Before it was 0 by default for main and 20000 for test and regtest.
Now it is 0 by default for all chains, thus there's no need to call Params().

Also now the default for main is properly documented.

Suggestion for release notes:

-fallbackfee was 0 (disabled) by default for the main chain, but 20000 by default for the test chains. Now it is 0 by default for all chains. Testnet and regtest users will have to add fallbackfee=20000 to their configuration if they weren't setting it and they want it to keep working like before.

Should I propose them to the wiki for the release notes or only after merge?

For more context, see https://github.com/bitcoin/bitcoin/pull/16402#issuecomment-515701042

ninja check check-functional