Reasoning: 10.10 has been unsupported since July 2017 (~3 years at v0.20.0 release) 10.11 has been unsupported since July 2018 (~2 years at v0.20.0 release) macOS users are consistent at upgrading to new releases. Qt 5.12 LTS only supports macOS > 10.12. As long as we're supporting macOS < 10.12 we would not be able to bump Qt in depends to 5.12 for the v0.20.0 release.
Backport of core PR17550.