Pretty much all of the older devices do still exist within newer versions of Renoise, purely for backwards compatibility reasons, but some of the older devices are technically not that great, which is why the decision is sometimes made to hide them if a newer device comes along to do a better job…
If you’ve got a song that was using the old Renoise 2.8 Filter device, and you load it into a newer version of Renoise 3.*, then Renoise will still load that older legacy DSP effect for you…
So from there you could actually save that old Filter device as a preset and then reload it into new songs, if that’s really what you prefer. (Or even copy/paste the device between multiple difference instances/versions of Renoise.)
There are a few forum posts discussing these old “deprecated” or “legacy” FX, such as:
( Edit: and of course the tool mentioned above by @slujr )
As I said, we typically only hide old stuff when a newer/better version comes along to fill the spot, but you can still find some sneaky way to use the old stuff if that’s really what you want (for whatever reason) 