I don't think political t-shirts are protected under discrimination laws. Businesses can deny people service as long as it's not legally discriminatory...see: No shirt, no shoes, no service. Lawsuit is unlikely to get far beyond them offering a refund or cheap tickets as settlement.

Very likely they will use the flight attendant as a scapegoat and put her on paid leave for a bit.

TLDR: Boycott Delta if you want, but they are unlikely going to be affected much by this.