[shirogane86x]

I always felt like it was kind of the other way around? Speaking from experience , most languages where immutability is the default and/or the only option can usually simulate mutability or have it as a stdlib-provided escape hatch (elixir and haskell come to mind), but most languages that are mutability-first really struggle to provide any sort of immutability that is reliable.

[aatd86]

At a datastructure level, perhaps, although functional datastructures are pretty fringe. (cf. Okasaki) In general, they can still be implemented by traditional imperative languages (obviously).

At the type/variable level, it's the opposite. We see it with const. Once it's in here, it's in there.