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.
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.