I was going to say "your custom content", but I don't want the Workshop flooded with OC ponies either. (Although nothing is stopping you from uploading them.)
Seriously though, we need the rest of the minor/background ponies for the new version of Garry's Mod. Sure we can still access them in the older version (if you have it) but the point is we need to move them over; it's not backwards compatible. Meaning you can't just upload anything to your addons folder anymore, everything must be from the Steam Workshop.
This message doesn't apply to My Little Pony either. At least 70% of the content of the old download page needs to be transported over to the steam workshop. [link]
You are not making an "old version exclusive", I would give you an internet slap for thinking that way. We have a lot of catching up to do!EDIT:
Okay, after testing a bit, I have come across the realization that I am a fucking idiot and figured out that porting the old models over is NOT impossible as I originally thought it was, it's just done in a very different way. Regardless, you could still upload to the Steam Workshop if you guys want to; it's always an option and there's no drawbacks. The reason I don't do it is because I don't want to upload someone's work without permission.