Putting things like "OT" or "POL" in the
subject line works for a little while but then gradually becomes
ineffective.

I think we just forget. We can all be more aggressive in changing subject
lines. How does this affect (or break) threading?

Changing the subject line (like by adding OT or POL) does not change the
threading in any way. The key is that the people participating in the argument
have to be the ones to add it, or they just skip your changed subject post.
--
Jim in NC