Fedora will not get Plasma 6 until 40 releases next month in April.

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

It is planned for F40, which will be there in 1-2 Months.
BUT, I already use F40 beta (Edit: alpha) on Atomic KDE.

Just install Kinoite normally, and then rebase to fedora:fedora/40/x86-64/kinoite (or however it is spelled) after typing rpm-ostree remote refs fedora and selecting the right variant.

I personally had a good experience in the last days with it and it feels absolutely usable.
Maybe wait until the stable release, then you can use uBlue, which gives you a saner configuration for the start.

I'm running kde6 on fedora rawhide kinoite, it's pretty stable.

Minor point of clarification though, kinoite isn't immutable ootb, it's atomic (which is to say it either fully updates or doesn't. Immutable is an experimental setting you can enable though)