I believe it is planned for Fedora 40 Kinoite: https://fedoraproject.org/wiki/Releases/40/ChangeSet#KDE_Plasma_6

There seems to be a Fedora 40 Kinoite ref available:

$ ostree remote refs fedora | grep kinoite | grep x86_64
fedora:fedora/35/x86_64/kinoite
fedora:fedora/35/x86_64/updates/kinoite
.....
fedora:fedora/40/x86_64/kinoite
fedora:fedora/40/x86_64/testing/kinoite

So you could rebase to it using: $ rpm-ostree rebase fedora:fedora/40/x86_64/kinoite

PSA: I have not tried this ref myself. This also assumes you will be running on a x86_64 based system.

Edit: I just tried the above in a VM and can confirm that the Fedora 40 Kinoite ref does indeed have Plasma 6 running. Fedora obviously does not consider this ref stable yet though