In your example (change a function signature and change all of its callers in one commit), would it require code reviews and approvals from every affected projects?
Yes, although if you can prove convincingly that it's a 100% safe change you can often get approval from someone who has approval power for most of the repository.
In practice, if you're changing 100+ individual callsites, then you would probably make a backwards-compatible change, then use an automated system to send out and manage a bunch of different commits to clean up call sites, then clean up your old function signature once all the commits are submitted. If your code isn't a really widely used library then you probably have fewer than 20 callsites, though, so it's nice to be able to do it in one commit.
In practice, if you're changing 100+ individual callsites, then you would probably make a backwards-compatible change, then use an automated system to send out and manage a bunch of different commits to clean up call sites, then clean up your old function signature once all the commits are submitted. If your code isn't a really widely used library then you probably have fewer than 20 callsites, though, so it's nice to be able to do it in one commit.