* the go module proxy ensures repos are never deleted, so everything continues to work
* changing to a new dep is as easy as either a replace directive or just find and replace the old url
If a module author deletes or relocates their module, the old module is not deleted or renamed from the module proxy. It is kept around forever. Code that depends on it to not break does not break.
If they relocated and you want to update to the new location, perhaps for bug fixes, then you do have to do a bit extra work (a find and replace, or a module level replace directive) but it’s a rare event and generally a minor effort, in my opinion, so I don’t think this is a significant flaw.
For most users most of the time they don’t need to think about this at all.
You're on a tear in this thread being wrong about how Go works, but I'm really curious what extremely specific series of events you're imagining would have to happen to lead to this outcome. If I use a dependency, it gets saved in the module proxy, I can also vendor it. You would have to, as a maintainer, deliberately try to screw over anybody using your library to accomplish what you describe.
As for Go, good luck doing just run when a code repo breaks all those URL hardcoded in source code.