Originally, NeverThrow/Ok and NeverThrow/Err were @psalm-immutable to allow usage of @template-covariant on them. It turned out that @psalm-immutable was too strict: all properties must be readonly which is good, but all methods must be mutation-free, which in this specific case is bad.

Ok|Err methods will cause side-effects most of the time, they are meant to cause side-effects: Database queries can fail, HTTP calls can fail, etc. Pure functions cannot really fail, at least not in the same way. Forcing all calls to andThen(), orElse() or match() to be mutation-free was a bad idea, so we decided to remove the immutable constraint on Ok|Err (we did keep the inner properties read-only). As a result, we cannot have @template-covariant which means the types don't "collapse" to Ok<A | B> but stay Ok<A> | Ok<B>. We can live with that.

map() and mapErr() were kept mutation-free, but it turns out it's just as painful and hard to use. We should also relax the constraint on those methods too.