Yeah, it's a slur. A High Court judge recently stated, "I have become familiar with the term TERF. It is a derogatory term used by those who seek to deplatform those who hold different views."
When a ton of hate, rape and death threats are thrown around, using a term? It's a slur.
When useage is assessed linguistically, the same terms surround it as are used for eg Jewish or black people by violently bigoted trolls. That's an extremely clear indicator.
‘Terf’ quite often shows up in the same tweet as other words whose status as sexist slurs is not disputed, like ‘bitch’ and ‘cunt’. Other words that occur include ‘disgusting’, ‘ugly’, ‘scum’ and a cluster of words implying uncleanness (‘smell’, ‘stink’, ‘garbage’, ‘filth’)—which is also a well-worn theme in racist and anti-Semitic discourse. Likewise the sexist insult feminazi is also used frequently.
People who would be genuinely furious if anyone suggested that they use all manner of other hateful terms happily use this one. It's only about feminists, right? Who cares if people threaten to rape or kill or slice their spines open. They're only women with opinions.
As is the fact it was created by others and used solely by those who hate the targeted and named group or person. It's used as a means of control and silencing and implicit ordering that any associated views are worthless, by those with narcissistic seeming behaviours that, to many women, look utterly male.
Finally, some try to argue that it's not a slur because it's descriptive. But firstly, SCOPE used to be called The Spastics Society, and an extremely unpleasant abbreviation of transsexual exists which may once have been nothing but descriptive, but is now a slur, so clearly that's not definitive in and of itself... and secondly, the word was created by a feminist who vehemently disagreed with the gender critical position. A word coined by an opposing side should not be used to label the other, while they protest, and then an innocent face made and, "but it's just descriptive!"
It was always meant derogatorily. It's never been used by gender critical people. And the hatred attached to it is plain. Slur.