The point of Nitter is that it uses the API their official web client (which is a JS SPA) is using. They can't lock out Nitter without also locking their own frontend out.
Twitter can alter that API at any point, since they can update their front-end at the same time.
Not possible as long as the pages are not walled behind a login.
They seem to be pretty stable when it comes to their APIs. Over 5 years ago I built a small service to provide your personal Twitter main timeline as an RSS feed. Never touched it since. Still works.