>>/1048805/
Filtering rules kept on the user's system/newsreader and never uploaded anywhere, that were usually just a list of rules/regexs for removing (or sorting up|down) according the author, subject, post size (back then==binary posts), etc.
I've thought that for an imageboard, accessing the filter editor would be a small button at the end of every post's header line (after "del"), and would bring up a uMatrix-esque editor, where a row of preset buttons for common filters (kill by userID, kill by keyword, ignore replies, hilight replies, ..)
There's been filtering systems like it since newsreaders were around, but they weren'y very capable or easy to use.