The National Labor Relations Act (NLRA) protects the right to wear union insignia at work.
Recently, it was ruled that they have to be small and "not distracting" where customers can see them. https://www.laborrelationsupdate.com/nlra/buttoning-up-rules-on-union-insignia-board-makes-it-easier-for-employers-to-restrict-size-and-scope-of-union-buttons-for-those-with-customer-contact-work/

That would be awesome, if you find out please let me know. Workers solidarity is the best, and we need so much more of it.

From the answers above I’m sure it’s allowed but are you working in a unionized Starbucks? I think I’d worry more about your SM and local leaders keeping an eye on you (they might think you’ll start wanting to organize a union) but of course that’s illegal. Just be careful, maybe use it someplace concealed where you can easily move and avoid a close look by people you don’t trust.

Yes it's allowed within reason. You can't put it in 60 point font covering up the siren on your apron, but something similar in size to the starbucks issued pins is fine.