diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -132,21 +132,17 @@
 If code formatting tools do not install automatically on your system, you
 will have to install the following:
 
-On Ubuntu 20.04:
+Install all the code formatting tools on Debian Bullseye (11) or Ubuntu 20.04:
 ```
-sudo apt-get install clang-format-11 clang-tidy-11 clang-tools-11 python3-isort python3-autopep8 flake8 php-codesniffer yamllint
+sudo apt-get install clang-format-11 clang-tidy-11 clang-tools-11 python3-autopep8 python3-pip flake8 php-codesniffer shellcheck yamllint
+pip3 install isort>=5.6.4 mypy>=0.780
+echo "export PATH=\"`python3 -m site --user-base`/bin:\$PATH\"" >> ~/.bashrc
+source ~/.bashrc
 ```
 
 If not available in the distribution, `clang-format-11` and `clang-tidy-11` can be
 installed from <https://releases.llvm.org/download.html> or <https://apt.llvm.org>.
 
-On Debian (>= 10), the clang-11 family of tools is available from the https://apt.llvm.org/ repository:
-```
-wget https://apt.llvm.org/llvm.sh
-chmod +x llvm.sh
-sudo ./llvm.sh 11
-```
-
 For example, for macOS:
 ```
 curl -L https://github.com/llvm/llvm-project/releases/download/llvmorg-11.0.0/clang+llvm-11.0.0-x86_64-apple-darwin.tar.xz | tar -xJv
@@ -154,17 +150,6 @@
 ln -s $PWD/clang+llvm-11.0.0-x86_64-apple-darwin/bin/clang-tidy /usr/local/bin/clang-tidy
 ```
 
-If you are modifying a python script, you will need to install `mypy` and `isort`.
-The minimum required version for `mypy` is 0.780, because the previous ones are
-known to have issues with some python type annotations.
-On Debian based systems, this can be installed via:
-```
-sudo apt-get install python3-pip
-pip3 install isort==5.6.4 mypy==0.780
-echo "export PATH=\"`python3 -m site --user-base`/bin:\$PATH\"" >> ~/.bashrc
-source ~/.bashrc
-```
-
 If you are modifying a shell script, you will need to install the `shellcheck` linter.
 A recent version is required and may not be packaged for your distribution.
 Standalone binaries are available for download on
@@ -174,7 +159,6 @@
 if another version is already installed, make sure the recent one is found first.
 Arcanist will tell you what version is expected and what is found when running `arc lint` against a shell script.
 
-
 If you are running Debian 10, it is also available in the backports repository:
 ```
 sudo apt-get -t buster-backports install shellcheck
diff --git a/contrib/docker/bitcoin-abc-dev/Dockerfile b/contrib/docker/bitcoin-abc-dev/Dockerfile
--- a/contrib/docker/bitcoin-abc-dev/Dockerfile
+++ b/contrib/docker/bitcoin-abc-dev/Dockerfile
@@ -8,7 +8,7 @@
 # it after it's done
 RUN mkdir -p /install
 WORKDIR /install
-RUN /bitcoin-abc/contrib/utils/install-dependencies.sh
+RUN /bitcoin-abc/contrib/utils/install-dependencies-buster.sh
 
 WORKDIR /bitcoin-abc
 RUN rm -rf /install
diff --git a/contrib/teamcity/setup-debian-buster.sh b/contrib/teamcity/setup-debian-bullseye.sh
copy from contrib/teamcity/setup-debian-buster.sh
copy to contrib/teamcity/setup-debian-bullseye.sh
--- a/contrib/teamcity/setup-debian-buster.sh
+++ b/contrib/teamcity/setup-debian-bullseye.sh
@@ -7,7 +7,7 @@
 TEAMCITY_DIR=$(dirname "$0")
 
 # Install all the build dependencies
-"${TEAMCITY_DIR}"/../utils/install-dependencies.sh
+"${TEAMCITY_DIR}"/../utils/install-dependencies-bullseye.sh
 
 # Python library for interacting with teamcity
 pip3 install teamcity-messages
diff --git a/contrib/teamcity/setup-debian-buster.sh b/contrib/teamcity/setup-debian-buster.sh
--- a/contrib/teamcity/setup-debian-buster.sh
+++ b/contrib/teamcity/setup-debian-buster.sh
@@ -7,7 +7,7 @@
 TEAMCITY_DIR=$(dirname "$0")
 
 # Install all the build dependencies
-"${TEAMCITY_DIR}"/../utils/install-dependencies.sh
+"${TEAMCITY_DIR}"/../utils/install-dependencies-buster.sh
 
 # Python library for interacting with teamcity
 pip3 install teamcity-messages
diff --git a/contrib/utils/install-dependencies.sh b/contrib/utils/install-dependencies-bullseye.sh
rename from contrib/utils/install-dependencies.sh
rename to contrib/utils/install-dependencies-bullseye.sh
--- a/contrib/utils/install-dependencies.sh
+++ b/contrib/utils/install-dependencies-bullseye.sh
@@ -14,14 +14,21 @@
   bsdmainutils
   build-essential
   ccache
+  clang-11
+  clang-format-11
+  clang-tidy-11
+  clang-tools-11
+  cmake
   curl
   default-jdk
   devscripts
   doxygen
   dput
   flake8
+  g++-9
   g++-aarch64-linux-gnu
   g++-arm-linux-gnueabihf
+  gcc-9
   gettext-base
   git
   golang
@@ -34,7 +41,7 @@
   jq
   lcov
   less
-  lib32stdc++-8-dev
+  lib32stdc++-10-dev
   libboost-all-dev
   libbz2-dev
   libc6-dev:i386
@@ -61,6 +68,7 @@
   make
   ninja-build
   nsis
+  pandoc
   php-codesniffer
   pkg-config
   protobuf-compiler
@@ -73,6 +81,7 @@
   qemu-user-static
   qttools5-dev
   qttools5-dev-tools
+  shellcheck
   software-properties-common
   tar
   wget
@@ -90,29 +99,11 @@
 apt-get update
 DEBIAN_FRONTEND=noninteractive apt-get install -y $(join_by ' ' "${PACKAGES[@]}")
 
-BACKPORTS=(
-  cmake
-  shellcheck
-)
-
-echo "deb http://deb.debian.org/debian buster-backports main" | tee -a /etc/apt/sources.list
-apt-get update
-DEBIAN_FRONTEND=noninteractive apt-get -t buster-backports install -y $(join_by ' ' "${BACKPORTS[@]}")
-
-
-# Install llvm and clang
-apt-key add "$(dirname "$0")"/llvm.pub
-add-apt-repository "deb https://apt.llvm.org/buster/   llvm-toolchain-buster-8  main"
-add-apt-repository "deb https://apt.llvm.org/buster/   llvm-toolchain-buster-11  main"
-apt-get update
-
-LLVM_PACKAGES=(
-  clang-11
-  clang-format-11
-  clang-tidy-11
-  clang-tools-11
-)
-DEBIAN_FRONTEND=noninteractive apt-get install -y $(join_by ' ' "${LLVM_PACKAGES[@]}")
+# Use gcc-9/g++-9 by default so it uses libstdc++-9. This prevents from pulling
+# the new pthread_cond_clockwait symbol from GLIBC_30 and ensure we are testing
+# under the same condition our release it built.
+update-alternatives --install /usr/bin/gcc gcc "$(command -v gcc-9)" 100
+update-alternatives --install /usr/bin/g++ g++ "$(command -v g++-9)" 100
 
 # Use the mingw posix variant
 update-alternatives --set x86_64-w64-mingw32-g++ $(command -v x86_64-w64-mingw32-g++-posix)
@@ -129,12 +120,6 @@
 # shellcheck source=/dev/null
 source ~/.bashrc
 
-# Install pandoc. The version from buster is outdated, so get a more recent one
-# from github.
-wget https://github.com/jgm/pandoc/releases/download/2.10.1/pandoc-2.10.1-1-amd64.deb
-echo "4515d6fe2bf8b82765d8dfa1e1b63ccb0ff3332d60389f948672eaa37932e936 pandoc-2.10.1-1-amd64.deb" | sha256sum -c
-DEBIAN_FRONTEND=noninteractive dpkg -i pandoc-2.10.1-1-amd64.deb
-
 # Install npm v7.x and nodejs v15.x
 curl -sL https://deb.nodesource.com/setup_15.x | bash -
 apt-get install -y nodejs
diff --git a/contrib/utils/install-dependencies.sh b/contrib/utils/install-dependencies-buster.sh
rename from contrib/utils/install-dependencies.sh
rename to contrib/utils/install-dependencies-buster.sh
diff --git a/depends/README.md b/depends/README.md
--- a/depends/README.md
+++ b/depends/README.md
@@ -65,7 +65,7 @@
 
 Then install the dependencies:
 
-    sudo apt-get install lib32stdc++-8-dev libc6-dev:i386
+    sudo apt-get install lib32stdc++-10-dev libc6-dev:i386
 
 For linux ARM cross compilation:
 
diff --git a/doc/build-unix.md b/doc/build-unix.md
--- a/doc/build-unix.md
+++ b/doc/build-unix.md
@@ -62,9 +62,9 @@
 ----------------------------------------------
 Build requirements:
 
-    sudo apt-get install bsdmainutils build-essential libssl-dev libevent-dev lld ninja-build python3
+    sudo apt-get install bsdmainutils build-essential cmake libssl-dev libevent-dev lld ninja-build python3
 
-**Installing cmake:**
+**Installing cmake on older distribution (Debian < 11 or Ubuntu < 20.04):**
 
 On Debian Buster (10), `cmake` should be installed from the backports repository:
 
@@ -72,11 +72,7 @@
     sudo apt-get update
     sudo apt-get -t buster-backports install cmake
 
-On Ubuntu 20.04 and later:
-
-    sudo apt-get install cmake
-
-On previous Ubuntu versions, the `cmake` package is too old and needs to be installed from the Kitware APT repository:
+If the `cmake` packaged version is too old it can be installed from the Kitware APT repository:
 
     sudo apt-get install apt-transport-https ca-certificates gnupg software-properties-common wget
     wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | sudo apt-key add -
diff --git a/doc/build-windows.md b/doc/build-windows.md
--- a/doc/build-windows.md
+++ b/doc/build-windows.md
@@ -5,7 +5,7 @@
 
 The options known to work for building Bitcoin ABC on Windows are:
 
-* On Linux, using the [Mingw-w64](https://mingw-w64.org/doku.php) cross compiler tool chain. Debian Buster is recommended
+* On Linux, using the [Mingw-w64](https://mingw-w64.org/doku.php) cross compiler tool chain. Debian Bullseye is recommended
 and is the platform used to build the Bitcoin ABC Windows release binaries.
 * On Windows, using [Windows
 Subsystem for Linux (WSL)](https://msdn.microsoft.com/commandline/wsl/about) and the Mingw-w64 cross compiler tool chain.