Noting that this question has been answered already 
, I will add for anyone interested that there is currently a blocker to upgrading Plasma past 5.16 – see PR#71232 for details.
(p.s. As I know that packaging desktop environments isn’t an easy task, I’d like to also give a big thank-you to everyone involved in doing so for NixOS. Sincerely, an appreciative KDE user.)