Imagine if anyone could remove their contribution to the linux kernel at any point.

There's a reason people can't remove their contributions to the kernel. Contributions to the Linux kernel are licensed as GPL which is not really revocable (GPL v3 clarifies this). You can't change your mind because the license you grant for your contribution gives you no means to.

User-generated content on websites like NexusMods have various licenses. To get around this, NexusMods explicitly states in the ToS that submitting UGC grants an irrevocable license to use that content, which is why they can say "no" to removing mods.

However, this was not always the case. The license I granted when I submitted my first mod, for example, was not irrevocable. For me, this was irrelevant because my code was licensed under MIT anyway, but had I uploaded the mod with the expectation of being able to revoke the license at any time, I'd have been pissed if NexusMods decided to disregard my request to remove the mod.

That's not quite the same thing.

Yes, someone can fork your project, but you maintain control over your own repository. Nobody gets to say "I need this, so you can't remove it or previous commits related to it from your repo". You can totally filter and scrub your own repos.