They have not done that for years? That used to be a thing 5 years ago or so, but nowadays battery saving mode just forces 60 hz refresh rate but otherwise does not change the resolution. I do not buy top of the line devices so I do not know for sure, but mine has >1080p and keeps the resolution for sure.
And, to give an idea of what are we talking about, even in such power saving modes these much smaller and older phones still have more pixels than a 1080p screen.
If you put the phone in battery saver mode, lowering resolution is one of the things it does.