There are 45 questions tagged forgery; nearly all of them are also tagged either mac, signature, or authenticated-encryption, or tagged with a specific scheme like elgamal-signature—see search for exceptions.
The standard security property of MACs and signatures is unforgeability under chosen-message attack, which is entailed by authenticated encryption, so the concept of ‘forgery’ is implied by the alternative tags.
What purpose does the forgery tag serve? Should we delete it?