[glowcoil]

The things you are saying are just not true. Standard ML and OCaml are in fact single-threaded languages, and while Haskell has concurrency and parallelism support it does not have anything that looks particularly like a Send or Sendable trait.

[pjmlp]

Concurrent ML:

http://cml.cs.uchicago.edu/pages/sync-var.html

As for the rest I am on the go to type a proper example.

[glowcoil]

Yes, Concurrent ML exists. It also, however, does not have any feature analogous to Rust's Send trait or Swift's Sendable protocol.

[pjmlp]

Of course it doesn't have a type class Sendable, that is not the point, rather the type theory behind it, and what those languages allow for, regardless of what they have in 2021 on their standard library.