I think you misunderstand what I'm getting at. SpamAssassin is older than Gmail. It's an old example, much newer and better spam-filtering-at-scale solutions exist (although SA is still maintained). Trying to claim that only the big boys can filter spam is an uninformed opinion.

No, you don't need an AI model to detect AI content (lmao). Heuristics already exist, and you see people mention them online all the time -- excessive use of lists, em dashes, common phrases, etc. Yes, a basic text heuristic scorer from the 1980s can pick these up without much difficulty. The magic of auto-learning heuristics (which have also existed since the 1980s, and performed fine at scale with less processing power than your smartwatch) is you can train them on whatever content you don't want to receive: marketing, political content, etc. You can absolutely apply this to whatever content suits your fancy, and it doesn't really take any more effort than moving messages you want filtered out to a Junk folder or similar.