TL;DR
Over the last years, the the main developer of Nitter, Zed, worked really hard to keep the project going. But Twitter worked just as hard on closing their network down and limiting any access they can't monetize.

Most Nitter servers were using a technique of generating loads of temporary tokens that were used for accessing the content, but that path is now blocked as well.

What to do now?
Don't trust corporations, especially those where one egomaniac has all the power. Use open-source and community driven solutions if you can.
https://nitter.cz/