Literally every single UK government entity, town/city council, every public transport company, etc. They all use it sadly.

Any specific examples where twitter was the only way?

A good example would be Supertram - the Sheffield tram service. It's only ever provided service disruption notices on twitter. The stations have digital signs which they installed years ago and have always been completely flawed in that they cant actually update them remotely, nor can they say if a tram is running late. They just play a fixed playlist on a loop.

As an anecdote from the States: During July 4th our city had some poor weather and the best way to check the status of any celebrations going on was their Twitter page.

Both Anthropic and OpenAI will sometimes make important announcements only on Twitter.