Yes, manifests are julia version-specific (although this isn’t really well-known ime), but pinning package versions for better reproducibility can be done with Project.toml alone.
I wanted not to conflate different meanings of “compatibility” and only talked about “update julia - keep all code (including package code) the same”. Sure, there are other scenarios like “a new package version should only compatible with new julia” but this is an independent story.
Again, this is a completely different meaning of compatibility, not the one I’m talking about.
This one seems easier to fix, you just need better tooling to test lower bounds. There is some, but it’s not polished nor advertised, eg:
Meanwhile, “Julia updates breaking existing package code” doesn’t seem easily fixable.