All extension branches were removed during the migration to Git. Very
few extensions have branches for MW core major version support.
There's no longer a simple way to branch all extensions when a core
release is updated, and nobody has volunteered to write a script.
So we're back to the situation we had in MW 1.9 and earlier, where
it's usually not possible to run any actively maintained extension
against an MW core that's not the current trunk.
Given this, I think code reviewers should insist on backwards
compatibility with MW 1.20 for commits to the master branch of
extensions that are commonly used outside Wikimedia, at least until
the release management issue is solved.
-- Tim Starling