Twitter is not dead; it rather caters to a different and broader audience segment now.

What happened is that it is no longer an institutionally governed "safe space" where only leftist progressive views are allowed to be voiced in public, and any dissenting views were censored.

The segment of people who think that applying that sort of leftist ideological purity filter on all public discourse is a good thing are leaving, while another (and ultimately much larger) segment of the public who highly value free speech are joining.