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Theyfab isn't a transmasc-specific slur. It's always been used against any nonbinary person assumed to be AFAB.
Though the AGAB of nonbinary people is nobody's business in the first place, it bears repeating that not every AFAB nonbinary person is transmasculine, just as not every AMAB nonbinary person is transfeminine.
These bigots aren't just transphobic towards trans men/mascs, they're exorsexist as well. We'll be stronger if we stick up for each other and push back against them together!
Edit: Just to make it clear, theyfab is being used against transmascs as well! I just want people to also acknowledge the non-transmasc nonbinary people being hurt.