A 20k LOC PR isn’t reviewable in any normal workflow/process.

The only moves are refusing to review it, taking it up the chain of authority, or rubber stamping it with a note to the effect that it’s effectively unreviewable so rubber stamping must be the desired outcome.

Dumping a huge PR across a shared codebase wherein everyone else also has to deal with the risk of you monumental changes is pretty rude as well, I would even go so far as to say that it is likely selfishly risky.

Dumping a 20k LOC PR on somebody to review especially if all/a lot of it was generated with AI is disrespectful. The appropriate response is to kick that back and tell them to make it more digestible.

Opening a 20k LOC PR is an actively hostile move worthy of an appropriate response.

Closed > will not review > make more atomic changes.

If somebody did this, it means they ignored their team's conventions and offloaded work onto colleagues for their own convenience. Being considered rude by the offender is not a concern of mine when dealing with a report who pulls this kind of antisocial crap.

I'm the owner of some of my work projects/repos. I will absolutely without a 2nd thought close a 20k LoC PR, especially an AI generated one, because the code that ends up in master is ultimately my responsibility. Unless it's something like a repo-wide linter change or whatever, there's literally never a reason to have such a massive PR. Break it down, I don't care if it ends up being 200 disparate PRs, that's actually possible to properly review compared to a single 20k line PR.