There's no Thread in core mruby (you have to go through a mrbgem[0]), so I don't think there is Ractor stuff in there.

[0]: https://github.com/mattn/mruby-thread

EDIT: confirmed

    $ nix-shell -p mruby
    nix$ mirb
    mirb - Embeddable Interactive Ruby Shell

    > RUBY_VERSION
     => "3.1"
    > Ractor
    uninitialized constant Ractor (NameError)

I believe mruby branched at Ruby 1.9