Ironically, the 1st Amendment protects Twitter's ability to block anything that it likes.

Fundamentally, the government can't dictate what anybody chooses to publish (or not) on a website that they own. They can seize it and prosecute the owners if the content is illegal, but they can't force editorial decisions.

This whole arrangement drove a lot of traffic to Twitter and helped to keep it relevant; they just didn't want to kill their golden goose.

Nope, no laws like that here. This was a Twitter decision.

> Pretty sure that was a point early in his presidency where he was block, but Twitter being a US company was not allow to block the US president....?

IIRC, there was a lawsuit that ruled that the president was not allowed to block other twitter users. As far as I know, it would still be totally withing twitter's right to block Trump, if it wanted. But I think they don't because what he says on Twitter reveals important information about his character.