it doesn't. never prevents reading and readonly prevents (over)writing.

GP just focused on the never-part of your question.

never means nothing can be assigned to it, so in that sense it's already readonly

Correct, so the question should have been: is readonly necessary?

I wouldn’t call myself an expert in TypeScript’s type system (although I use it daily), so I’m not sure about that one.

I do know I would have omitted readonly if I did this myself, but perhaps by mistake.