There were often "rate-limited" errors that hadn't completely prevented the site from being usable since the nitter people said the site would go down eventually.
Now however there is an official declaration on the site itself: https://nitter.cz/
Due to the reasons stated by Nitter itself on their repo, it seems unlikely there will alternatives.
Are there any currently?
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.