First and foremost, Linux Mint 21 has been dubbed “Vanessa” and it will come with the same three editions that you’ve been used to, featuring the Cinnamon, Xfce, and MATE desktop environments. So no, a KDE Plasma edition is not in the works for the next major Linux Mint release.
Second of all, as you might have expected, Linux Mint 21 will be based on the next major Ubuntu LTS (Long Term Support) series, Ubuntu 22.04 LTS (Jammy Jellyfish), which means that it will also be supported for a long period of time and that it will be powered by its Linux 5.15 LTS kernel series.