In the UK, we can cite several things to show TERF is considered to be hate speech:
District Judge John Woollard acknowledged 'Terf' as a derogatory word aimed at, primarily, gender critical women. (Basildon Magistrate's Court on 01/03/2019 REGINA vs YARDLEY).
All Party Parliamentary Group Report on Hate Crime stated that:
"It can easily be argued that this constitutes hate speech under Criminal Justice Act, which people have been successfully prosecuted for."
Standards Commission for Scotland again: Dundee Councillor Gregor Murray was suspended after the Commission found Murray "abused the complainer by referring to her as a TERF (a pejorative term which stands for ‘trans-exclusionary radical feminist’)."
Mr Justice Knowles in the Harry Miller vs College of Policing case acknowledged "terf" as a derogatory slur. That's UK case law. Miller won his case, if I recall correctly.
“I have become familiar with term TERF. It is a derogatory term used by those who seek to deplatform those who hold different views”. 21/11/19 UK
Judge also asserted, "It can be pejorative."
Also, Medical Journal of Australia has recognised 'terf' as a "derogatory term":
"Aug 8, 2019
The MJA apologises unreservedly for the derogatory term used in a recent post, which does not reflect the views of the MJA. A review of all our social media protocols is underway to ensure similar incidents are not a part of the MJA's social media activity - MJA Editor-in-Chief"