Imagine team A vendors into their repo team B's code and starts adding their own little patches.
Team B has no idea this is happening, as they only review code in repo B.
Soon enough team A stops updating their dependency, and now you have two completely different libraries doing the "same" thing.
Alternatively, team A simple pins their dependency to team B's repo at hash 12345, then just, never updates... How is team B going to catch bugs that their HEAD introduces on team A's repo?
This is already caught by multi-repo tooling like Github today. If you vendor in an outdated version with security vulnerabilities, issues are automatically raised on your repo. Team B doesn't need to do anything. It is Team-A's responsibility to adopt to latest changes.
Curious because I haven't seen this myself. Do you mean, GitHub detects outdated submodule references? Or, GitHub detects copy of code existing in another repo, and said code has had some patches upstream?
(In our org, we have our own custom Go tool that handles more sophisticated cases like analyzing our divergent forks and upstream commits and raising PR's not just for dependencies, but for features. Only works when upstream refactoring is moderate though)
Team B has no idea this is happening, as they only review code in repo B.
Soon enough team A stops updating their dependency, and now you have two completely different libraries doing the "same" thing.
Alternatively, team A simple pins their dependency to team B's repo at hash 12345, then just, never updates... How is team B going to catch bugs that their HEAD introduces on team A's repo?