I've seen plenty of people unironically wear Che Guevera T-shirts, Socialist t-shirts, etc. even at conferences. If that's not at least at little bit threatening to some people, let me introduce you to my friend who helped people escape over the Berlin wall.

Whether you like it or not, plenty of people who voted for Trump are probably in attendance. Telling them that they're unequivocally not welcome is hardly going to change their minds.