Shameless(-ish) plug for my project, siftrss: https://siftrss.com/
You add a source feed, include/exclude items by keyword or regex match, and get a new filtered feed to subscribe to. Because it produces a normal RSS feed like any other, it works with all RSS readers.
It's totally free. No ads, no account, no email capture, nothing but totally disposable RSS feed filters.
I haven't looked at their implementation much, but I imagine it only works on Feedly itself whereas siftrss is meant to be totally agnostic to what reader you use. The tool also supports regex, so you can do some slightly richer matching with siftrss.
Hmm, I wonder if there's a way to use it to turn a Medium feed into something useful. (By default, a medium feed is not, as it contains all comments by the author interspersed with actual stories/posts, with no obvious way for software to distinguish).
As for Medium feeds, it's all about whether or not you can find a pattern in the data to exploit. For instance, maybe links for comments contain "/c/" whereas links for posts contain "/p/" or something similar. There is, of course, no guarantee that there is anything to exploit or that what is exploitable won't change—that's the joy of dealing with third-parties! :)
That is a neat idea, though!