You may want to cross-post this to !lemmy@lemmy.ml instead, as that's where Lemmy-as-software discussions/requests are more appropriate. It'd be a nice feature for sure though, and the current "Hide read posts" option is just a bit too overkill to suggest as a workaround for you for now, I think.

also useful to hide post that contain a string in the title.

Seconded, would be nice to also have "hide all posts with this url" to cover posts to other communities/servers, but that's less critical.

This feature is available in some clients

It's title and body better for you than title + body? I added it to my app after multiple people asked for it, but I don't understand the appeal

You should give Voyager a try https://vger.app

You can slide posts to hide them, that's how my feed stays fresh.

Not quite the same thing but just today I've come across this userscript: lemmy-keyword-filter

It's a very simple keyword filter. I am using it to get rid of some noise from a community that I am otherwise interested in.

I use Violentmonkey to run it but any userscript engine should do as long as it supports GM_getValue, GM_setValue and GM_registerMenuCommand which are used to provide a minimal UI to edit the keyword list. You might want to edit the @match rule to restrict it to the Lemmy instances you use.