I think it's available in the $6/mo plan.. and as a paying customer of Feedly's -- I'll say it's worth it.
Feedly does it, as you mention.
There is an open feature request for NetNewsWire and apparently it sort of obeys Feedly's mute feature.
https://github.com/Ranchero-Software/NetNewsWire/issues/3018
You can do this kind of filtering using https://www.pipes.digital/ (spiritual successor to Yahoo Pipes) and then in your client app, subscribe to the Pipes feed instead of the upstream feed.
You can self-host miniflux, it has a content filter blacklist/whitelist (https://miniflux.app/docs/rules.html)
Yes, available in the Pro+ plan in Feedly https://blog.feedly.com/leo-and-mute-filters/
This would be useful, especially for filtering out authors in multi-author feeds (e.g., Volokh).
You’ve got a few options on the self-hosted front. TT-RSS and FreshRSS both have quite advanced filtering systems that allow you to highlight or mute articles.
Most of the paid options have similar functionality, including Feedly, NewsBlur & Feedbin.