whats to stop someone from running a s19 at 1% fan speed with a heavy underclock? it would be much more efficient compared to an older s9 when ran at 300W or something
Because an underclock S19 would still do 2300W which would release more heat than an S9 mining at full power, thus the fans are needed.
It's one thing to disperse 600W in a cold environment a different thing for 2000W.
And if you plan to strip it down to reduce consumption, why even do it, you're just throwing away money at a really expensive heater unlike what OP is doing.
the problem is, you can't currently slow down an S19 miner...
I do not know any firmware / software with which it is possible and it must first create the ROI and make a profit like an old S9

There is:
https://bitcointalk.org/index.php?topic=5428227.msg61466927#msg61466927