I'm not convinced that non-WG lists should be announced in a formal way, but it would certainly be dumb to create a list and tell nobody.
If it is ok to create a list under ietf.org, it ought to be ok to announce it on ietf-announce. If it is ok to announce such lists, it ought to be ok to announce similar lists that are running at other sites.
The question, then, is when it is ok to create a list at ietf.org. I'm guessing that the existing criteria are something like "the activity is targeting wg status and is beginning the process, through online discussion." If one wanted to be highly formal and objective, but still have the scope for such lists constrained to IETF activities, then requiring a draft charter and a BOF request, or the like, might be reasonable.
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