What happens of you rebase a branch containing commits with notes attached?

Notes are copied to from the original to the rewritten commit by default. See https://git-scm.com/docs/git-notes#Documentation/git-notes.t... for details.

But I have this in my IRC logs:

  < _jwilk> TIL git-notes rewriting doesn't work properly when doing amend within rebase. :/

Yeah I think that's a pretty big issue. Not going to be fun using git notes for code review if they get lost every time you squash or recreate commits.

One thing that would be really useful though - if you could somehow use git notes to tell Git not to download some blobs by default that would be great. It would solve the "someone added 100 MB of binary files to this project 5 years ago and then deleted them" problem.

That would be too useful though so I wont hold by breath.

Check the notes.rewrite config options in https://git-scm.com/docs/git-notes#Documentation/git-notes.t... Also notes.rewriteRef. You can use these to configure git to carry your notes across amend/rebase.