Steam Deck

It would work in desktop mode. All Decky plugins do, because Decky itself sets up system level hooks for the plugins. You can also access your Decky plugins by launching Steam in Big Picture Mode while in desktop mode.

But yes, the custom limiter would not trigger if the battery level crosses up past the threshold. It would continued to charge untill the stock behavior of tapering off past 90%.

It would continue to limit your charge if you were above the threshold before the Deck goes to sleep. It would also continue to "idle" the battery of you force it to idle regardless of the charge level. It's the custom charge threshold limit that won't trigger if the Deck goes to sleep while still under it.

I'm going to dump a walk of text trying to describe this with an example because I don't trust my ability to explain this any better otherwise— Realistically, if you, say, set the threshold to 60%. Watch a few shows with it plugged in until it ticks up to 60% and then go about your business, letting it go to sleep, it would not continue to charge your battery. A couple of days later you might return to the Deck at 50% charge or something, even though it is plugged in. And then it would start charging again once you wake it up. Assuming you keep it awake long enough to get it to 60% again. Surely with an hour of tv a week it'll cost under that 60% mark despite the limitations!

OR, you can just not bother with the limiter at all. Force the battery to be idle all the time. You intend to only use this Deck docked, unlike me or Stampela from the comments, you don't need to unplug the Deck and play a few games on battery.