Drop D tuning where you lower the E string a step but leave everything else the same does let you play one finger power chords. But Keith Richards doesn’t use that tuning as far as I know. You seemed to say that he used Drop D in your earlier post but maybe that’s not what you meant.
You also mentioned Open G tuning, which he does of course use and which involves tuning the low E string down to a D as well as retuning some of the other strings. But unlike Drop D tuning, Open G doesn’t allow a one finger power chord on the E and A strings. As you mentioned in your last post, in Open G those strings are a fourth apart, not a fifth apart, so you have to use the typical power chord fingering.
On top of that, KR likes to remove the low E string altogether when he uses Open G tuning, in which case there’s no way he’s doing any kind of power chords involving the low E string.
That leaves the A and D strings. He could theoretically do a one finger power chord on those strings when he’s in Open G tuning since, as you mentioned, they’re tuned a fifth apart. But he doesn’t do that as far as I know, I think mainly because one finger power chords aren’t really part of his or the Rolling Stones’ style. One finger power chords are more of a grunge and metal thing.
Or to put it another way, can you name a Stones song that features Keith Richards doing one finger power chords? I can’t think of one offhand.
Off the top of my head, I thought Keith Richards played one finger power chords all over ‘Start Me Up’.