Kinda don’t like how my handwavy idea is just taken for the most naive turn. I’m not even trying to give precise solutions. I’ve never worked with software at scale, and I expect the playing ground to be pretty different, but I think you’re exaggerating.

1. Storing all 18 years worth of data in all its iterations is ridiculous in the first place, and should never cross the mind of any dev worth their salt for more than a mere nanosecond. Cut off all that data down to to 3 years, 1 year, or even just a few months, and that’s probably all Reddit needs for backup and analytics. Have separate strategies for backup and analytics if needed. They’ve been doing ads and analytics stuff for a while now, so I expect them to have some architecture in place for that.
2. Dealing with deleted comments is easy — just unmark them for deletion (hard delete is generally not a thing). It’s most probably not in a backup. It’s just not a user accessible feature to unmark deletion. Even if they do get deleted eventually, what’s the time frame for a cleanup like? Every day? A few months? They still need an entry for that comment for the threads feature to work, so at best, they null the content of the comment out.
3. ChatGPT is just an example. No need to beat a bad example to death and use that as an argument against a whole argument. And I’m pretty sure you’ve not read the rest of the last comment.
4. I think you’re over-estimating how much of an impact the API pricing fiasco had, and once again, you don’t seem to have read my previous comment and acknowledged that. Nobody in their right mind is going to do this comment read and scan for every single Reddit user. Not manually for sanity. Not programmatically for cost. It’s why they need some way(s) to identify which users to watch out for. They’re not going to do that manually though, right? That would be costly too, from a manpower’s perspective, and human labor is expensive, and scales much worse than programs.
5. Common sense would ask that if all they did is to restore their database to a certain state, how do they deal with new comments and changes that were added between the PiTR and whenever they make the restore? Are they just gone now? Isn’t that bad, cause they’re potentially losing new, quality content?

Look buddy, all I want to say is that I don’t think your method against Reddit would work. It’s basically gamble though, so I’m definitely not against attempt at it. I just want to point out the possibility of it not working. I don’t think there are surefire ways against their attempt at restoring content.