Classical (aka “acoustic”) musicians - piano, quartet, what you will - perform (and record mostly) in untreated rooms. But they tend to be great rooms. Large rooms - sometimes Very Large. In country mansions and palaces and the like. And mostly with fanstatic wood floors and stages. The quality of these rooms is judged not by their relative deadness or lack of modes, but by the subjective judgement of the quality of sound when the instruments are played. It’s a naturally good sound, if not exactly uncoloured.
It’s not where we are though. Personally I’m convinced by the notion the best room would be a park sans chirping wildlife.
My recording spaces have all been pretty small and I only (and I mean “only”) record acoustic guitar and vox. I learned the hard way that sound absorption treatment was essential to me getting anything recorded that I would dare try to sell to somebody. It’s all about control. How could I think about EQ and reverb if I didn’t have something reasonable to start with.
For an afternoon’s entertainment I now go to music stores and watch and listen as folks attempt to judge the reletive sonic merits of expensive acoustic guitars in booths hardly bigger than a phone-box. Ha!
How much a mic picks up an untreated room?
It depends entirely where in the room the mics are.