It never did

First Amendment constraints don’t apply to private platforms, Supreme Court affirms https://t.co/ZwJ12SMPUA pic.twitter.com/z4vhmBbf3s

— The Verge (@verge) June 17, 2019

The issue the majority ruled on was whether the private party in this case had "stepped into the government's shoes."

5 Justices said it had not. Considering how Trump uses Twitter, the majority might switch on the question of Twitter and the 1st Amendment. Just sayin'....