Those special features all have a maintenance cost, and if no one is around to pay it then their usefulness will diminish until they reach the threshold where it's not worth it anymore. Not sure why any filtering algorithm would be considered out of the ordinary. Do you know how many scientific and mathematics packages I've seen that are are really old, outdated, and suffering bitrot? Quite a few. In fact that seems to be a field where the old algorithms are discarded fairly regularly in favor of new ones. The older algorithms that remain popular do tend to stick around.
Then some mechanism of deprecation could be immensely better than removal (in software with tolerance - an image viewer or an audio editor are not the same as medical equipment firmware or banking software). It could also encourage the interested to review and update the code in case of need.
Some pieces of software are more free to grow, their value is in options (image processing), while others have value in their reliability (banking).
This said, one phenomenon that appeared in the "unfortunate decade" and which is extremely dubious is that of the propaganda that "less is more". No it is not. Freedom is "more". Constraints are "less". (In the general reality of desktop software applications.)
We do automated computing to have tools in general, as we may need them; and we use special features when we need them.
Figure imagemagick discriminating options or functions the same way ("Lanczos stays in, but we could ditch Hamming").