So I did, multiple times, each time getting the same warning. It is now shorter than it used to be. The ToS say sigs can’t be more than 4 lines or 1000 characters, both of which my signature clearly is not (It is 3 lines and <300 chars). It doesn’t contain anything else that would violate the terms either.
What I have noticed is that if I remove one of the links from the signature (there used to be two links), the signature violation warnings cease. Adding it back in produces the warning message again. So it seems that the second link (which is to an SRK thread) is somehow violating the ToS? I’m a little confused, as I used to have both links in the signature, so there doesn’t appear to be a “maximum links in signature” rule.