It's easy to make a script which creates such branch for each extension,
but I'm not convinced that's the right thing to do.
Also, did we make Extension:Distributor properly support branches on git
extensions?
In an ideal world we would have perfect tests for all extensions, and
jenkins could automatically detect when a change makes it incompatible
with an old (but supported) MediaWiki.
Maybe a theme for a hackaton / Google programs / new volunteers.