You are correct that Ocaml has a ref type (though I believe it is actually a special case of a mutable record with one field). Most stuff I've run into uses mutable records more than refs (granted, I'm hardly deep into the Ocaml way of doing things). There is a small difference between an immutable record of which one property is a reference to changing data and a mutable record where the record itself changes. I prefer the first though I realize this is mostly preference.

2017 was just 3 years ago. Loads of Ocaml stuff rely on versions much, much older than that. In any case, the idea of outright changing a formerly mutable structure to an immutable one would be unthinkable in most languages due to all the breakages it is likely to cause.