Such filters will delay selected frequencies around the cutoff, but leave others untouched and won't change the levels of frequencies. Traditional uses are phasers and synthetic reverbs, where the frequency selective delays plus feedback can create ringings and phase cancellations (phaser) or can "smear" frequecies in time (reverbs).
As I've said, a single allpass filter can also change the shape of a waveform. That's why I've suggested trying it out on analog waveforms and looking at the scopes or sample data, like a sawwave will no longer look like a sawwave but will still sound the same, and when you put it through a distortion in those morphed states it can result in different harmonics generated compared to the plain state. Also transients can sound a bit different, click type noises like an attack of a bassdrum where the "click" impulse will be kind of "strechted in time" in subtle ways. I think this effect is very useful for altering sounds expecially when distortions are in the game. Maybe it might also help to kind of "compress" a waveform in a way that hard steps and peaks are weakened a bit without really compromising the way they'll sound to the ear.
After quick googling found nothing about this particular device in the past of renoise, maybe it wasn't "deprecated" but rather "hidden" all the time for pure dsp testing purposes. Like the faust device that I can't really find anything that could be done with it for - probably there's an abi in it, and it's for internal testing of dsp code also. It's not that grand find after all due to the limited usefulness, but I though maybe one or other might find it interesting to play with.