Wait. Sabbat versus sabbath? Are they interchangeable? Is it offensive?
Well, Sabbath is the english transcription of the Hebrew word שבת (shabbath) meaning rest. In case you're not familiar, Shabbat begins on friday evening and ends on saturday, and during this period of time we are not allowed to do many things. I think Sabbat is just derived from Sabbath, and the two are interchangeable in my opinion.
Is it offensive? Some would argue it is, some don't really mind. The association between witches and the sabbath definitely has antisemitic roots though. Jews were often depicted as demonic beings, having horns and tails, possessing evil magic powers and drinking the blood of Christian babies. We were accused of stealing consecrated hosts to stick pins in them. Goyim (non-Jews) even believed we worshipped Satan (probably because of the whole 'synagogue of Satan' thing in the New Testament) and that we made a deal with him to destroy the Good Christian Society™️ etc, etc. So Jewish people were conflated with witches, since we weren't Jesus-worshipping people.
I personally don't care if non-Jews use the term Sabbat/h in their practice, but that doesn't mean other Jewish people won't find it offensive.