Plenty of TERFs consider it a slur and negative. JK Rowling, for example, wrote: "...activists who clearly believe themselves to be good, kind and progressive people swarmed back into my timeline, assuming a right to police my speech, accuse me of hatred, call me misogynistic slurs and, above all – as every woman involved in this debate will know – TERF."

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.