No it doesn't.
If the content of a url changes then the only way to have reproducibility is caching.
You tell nix the content hash is some value and it looks up the value in the nix store.
Note, it will match anything with that content hash so it is absolutely possible to tell it the wrong hash.
Not having a required input, say when you try to reproduce a previous build of a package, is a separate issue to an input silently changing when you go to rebuild it. No build system can ensure a link stays up, only that what's fetched hasn't changed. The latter is what the hash in nix is for. If it tries to fetch a file from a link and the hash doesn't match, the build fails.
Flakes, then, run in a pure evaluation mode, meaning you don't have access to stuff like the system triple, the current time, or env vars and all fetching functions require a hash.
Buildkit has the same caching model. That's what I'm saying.
It doesn't force you to give it digests like nix functions often do but you can (and should).
