Note: I’m not an user of TT-RSS myself, but I have a blog that people with TT-RSS are reading.
It seems like even the most recent installs of TT-RSS that people use to read my blog don’t really cache feeds correctly. In particular it looks like they do not send a
If-Modified-Since header, which means Apache has no chance to return a 304 (Not Modified) status.
Please see https://ctrl.blog/entry/feed-caching for some of the caching information.