Does that warrant a warning? That's just a natural occuring discussion, and selecting threads to follow is basic forum-reading skill. The rare gems of surprising insight are often found in the depths of slightly off-topic discussions.
Previously I've noticed another kind of meta-comment, the top level complaint by a self-appointed moderators about the overall shape of the discussion elsewhere, instead of engaging directly with the actual posts in question.
With intellectual pursuits, you can always find diamonds in the rough. Simply due to the effort YOU are putting in, given your then state of mind, knowledge and experience.
You can wade through a conversation on creationism and evolutionary denial and understand humanity or even deepen your skills.
However that's you - your unique circumstances bringing more to the table.
Others would very much appreciate a warning, because they already have working definitions, have seen this argument before and would prefer spending their time on other pursuits.
With a little forum experience, who has trouble scanning a thread and seeing it meander away from their interest? So I question the need.
Even if there is the need, who should be the judge for others over the quality of a subthread? I would feel presumptuous to pass judgement beyond my vote for each comment.
Aren't those diamonds all that matter anyway? Recently I increasingly find new and curious beginnings dampened and limited by intellectually lazy attitudes, by comments that start with "I mean...", that reiterate the default state rather than entertaining a new thought. Inadvertently, well-meaning soft-moderation like yours might steer even more people away from interesting topics.
Previously I've noticed another kind of meta-comment, the top level complaint by a self-appointed moderators about the overall shape of the discussion elsewhere, instead of engaging directly with the actual posts in question.