Hi Eric, there's a general consensus that users should be allowed to go down to brightness 0 if either Plasma/PowerDevil/KWin can guarantee no breakage or the user has explicitly opted into it.
There is indeed a more recent MR that allows to get the old behavior: plasma-workspace!4117 and we've had a bit of discussion about how to implement it properly, leading to !351 (merged) here in PowerDevil which I just merged yesterday. A few more changes are necessary to expose this in the applet and/or KCM. Not sure if we can manage to cram it into Plasma 6.1 still, but that checkbox will be there for users that want brightness 0 also for laptop displays.
Generally though, in terms of concepts, I believe we want to use "display off" for completely dark screens, and any value on the brightness slider (including 0%) as not completely dark. The checkbox is merely an override and primarily meant for people whose screens are not turning off at brightness 0. If we want to make turning off screens easier, that would be a separate effort and probably a different toggle.