Wikipedia and stack overflow and forums like reddit and chat and similar are the biggest benefits of the internet and they are very cheap to run, you could run them based on donations. Reddit is more expensive than it has to be since they try to pivot to more ads and media, but a text forum is very cheap.

The biggest benefit from ad supported tech are search and video, the rest would be better without ads. Reddit would be a better place if they didn't try to get ad revenue etc, in those cases them chasing revenue makes user experience worse instead of better.