The term "TERF" is exclusively used by people who attack and criticise "TERFs". It's never said without hostility, and it's often accompanied by explicit calls

The term "TERF" is exclusively used by people who attack and criticise "TERFs". It's never said without hostility, and it's often accompanied by explicit calls for violence (try searching Twitter for "kill TERFs" and "punch TERFs".) So yes, it certainly is an offensive slur - those who use it invariably mean it an offensive way.