Spam filtering is a horrendous problem which can never be "solved" because people continue to innovate around the filters. It only looks "solved" because gmail have enough data to do an amazing job of it.

I have a gmail account and a non-gmail account, and the non-gmail privately hosted one gets considerable spam, some of which even makes it through my manually curated filters.

I used to self-host email but the minimum time and expertise investment is now too high for me - I got fed up of having to fix breakages or losing email from random other sources.