You can't honestly expect every single modder to know the subtle difference between these formats. Because for a non-artist, all you can judge from is file size, and JPG is very good at that!
You can't honestly expect every single modder to constantly check what kind of stuff the SCP removes on a daily basis, and revise their work accordingly. Updating, re-VPing, retesting and reuploading constantly is a lot of hassle, not to mention that because of the very frequent updates, there'll be numerous versions all around the Internet, assuming you don't have a control over all the mirrors.
You also can't honestly expect every single player to know that the SCP no longer supports JPG, and that's why the mod he's about to play doesn't work. I imagine that a lot of players would quite simply vent their frustration at the modders and rate their work 0/10 just because he can't run it.
Removing features is an irresponsible and self-centered move. You don't like JPG? Fine, then don't use it, but if someone from the SCP had bothered to implement JPG support, don't remove it.