This is, generally speaking, a very low volume list. It's also a list that has a significant number of people on it who are unlikely to reliably tag their posts (hell, I'm unlikely to reliably tag my posts by topic area.) And, as Risker pointed out, I think we'd be the first WMF list doing this. I may turn on topic filtering and forcibly change a subject header to make it work the next time there's a gigantic thread about a particular topic so that people can opt out of the giant messy threads that come up from time to time, but topic filtering before that seems both ineffective (in that most people won't tag their topics properly, and we certainly won't stop them from participating on the list for doing so) and like a lot of work (for those who actually tag their topics.) Deleting an email takes like half a second - deleting all emails about the English Wikipedia sent to this list in any given month, discounting the giant messy threads, probably takes something like a whole minute, heh.
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Kevin Gorman