Nitter Is Shutting Down Following Changes to X Guest Accounts
"Although some Nitter instances are currently still operational, it is anticipated they will cease functioning shortly as they expire. Nitter was a privacy-oriented alternative front-end for X/Twitter."
"It appears the project's termination is linked to X/Twitter's decision to disable the creation of guest accounts. Nitter was heavily reliant on these anonymous accounts, which were only supported by older versions of the Twitter app."
You can check which Nitter instances are still up and running here.