The MLs clearly define what is / is not offtopic Sorry Simon, they do not, and could not, because topicality is not the kind of concept that admits of a definition (in terms of individually necessary and jointly sufficient conditions). It'a concept that essentially depends on how someone interprets something in some context. Of course, you might try to *approximate* something like a universal definition -- until it runs into a counterexample and chokes.
To make explicit something implicit in my proposal above: there is a key difference between: - a flock of anonymous people being toxic on instant messaging channels; and - people who've known each other for weeks if not years and who have a propensity to knock each other's egos. Rubbing the latter under the brittle "on / offtopic" notion, and then adopting a "warn then mute" approach is losing a key opportunity to *actually solve the issue*. Instead the relational issue is just hidden and may come back stonger next time.
Involving the Board I am glad I nowehere implied that the idea of "treating the root cause of behavioural / relational issues" on the MLs or oS communication channels required the Board. You can have your "moderator" do the thing I am describing (helping two members overcome their animosity) and turn to the Board only if all other failsafes have failed. And it's probably for the better, since people from the Board are often -- from what I've observed -- involved in these problematic exchanges.
Now it occurs to me that the Board has already made a decision, so there is no point for me to argue against it. If the moderation team prioritizes the approach I am advocating over the lazy "warn and mute" solution, it can work. I hope it will.