Has anyone done this on s19 models?
No and it won't be for a long while. Unless you want to have a "space heater" that even downclocked using 2000-2500W of 220V power that still requires a deafening fan from both the PSU and miner itself at your feet. This is for old gear that you aren't trying to get massive hashrates.
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
I am not trying to argue with you, but first it is not possible to underclock it that low as far as I am aware. Second if you have ever used a Bitmain PSU you would know even that is note quiet which is why you see people using different ones than the supplied. Lastly the idea is to use the extra power (watts) it creates due to its inefficiency to produce heat to warm a room. It has NOTHING to do with what is more efficient.
I understand what you are getting at as far as if it were possible to do so with an S19, but you are talking about using a $2000+ miner to do something that isn't what the idea is for. People either already own S9's previously or buying them for $100-$200 to have a small heater due to waste heat from energy that can solo mine. It's just a little more expensive space heater for your feet/room that could possibly hit the btc lotto.