It is compressed, yes, but at higher bitrates it is actually usable for modems/faxes, you're limited to low baud rates anyway due to jitter and sample intervals. But really the benefit is 16k sampling rate instead of 8, I'm not saying it's great but it's the best we've got.

It would be possible to mask this with a relatively simple FXS device to hide all of this and pretend it's a modem while packetising the actual data, but I guess the demand is almost zero so why bother.

I've been looking for a solution for many years to retain dialin services without having racks full of modems and trunks that are rapidly going out of fashion and at some point will not be available except via IP (and now we have the same problem at both ends), but they just don't exist.