One very simple measure I hope they implement is just not sending emails for unverified spam like this. I’d argue a majority of issues or comments do not need instant emails. Even one hour delay could help in combating abuse like this if they had any sort of reasonable moderation rules.
Either you’re unlucky or I’m lucky, I’ve reported scammers to GitHub multiple times and always got a response in a couple of hours.
Usually it's a new user who clones a few repositories to pass whatever mitigation they have.
Always get a "lots of reports, this may take a while" email first though. I don't think I ever not got that one.
I think there's something to be said about sending - by default - user generated content by email automatically if you've replied once to a thread. Lots of bad defaults here imho.