It will immediately be reported to mods once someone post a ref link but wearing them under your signature is allowed if I'm not mistaken? Only if you do not participate in the eXch campaign, or does it also apply to non-participants?
You can have referral links in your signature, there are not many restrictions if it is not a question of prohibited services such as mixers for example.
Whether it is allowed as part of a paid signature usually depends on the rules of the campaign written by the manager. And here, too, it is most often ok to use the referral service code from the signature, but there are still exceptions. Still, in such cases, everything depends on the campaign manager and the conditions he has set.