This is a support question and belongs in !lemmy_support@lemmy.ml

Actually, a scriptable filter (JavaScript? Lua?) for individuals might be nice in a client app or web app

I don’t think there is any filtering abilities built into Lemmy as of yet, but some of the 3rd party apps may support something like this. I think Mlem on iPhone has a filter function where you can block keywords, but that will only work when accessing from within the app im pretty sure.