Nitter worked by signing in with special "guest accounts" that I think were given to fresh downloads of the mobile apps.[0] Last year, X fully disabled that functionality, which left most Nitter instances broken. At that point, the developer publicly abandoned/ended the project.
The couple of instances floating around that still work are, to my knowledge, forks of the original Nitter that have been upgraded to work with a pool of manually created X accounts, which is a relatively expensive and fragile approach that most instances probably aren't up for taking.
