It isn't just Twitter, it's every single website that's turned themselves into a login-walled "application".
Twitter's relative openness lasted a long time. It was open by default because it is a product built in 2006, when the idea of coralling people into walled gardens to show them ads didn't exist.
Apps built later take the concept of "walled garden" as a default feature. Slack , Discord, Snapchat, Tiktok, Telegram .... all largely closed off platforms. You can't see anything unless you're logged in.