I remember you. You probably don't remember me though. I joined a little bit before you left if I remember correctly. My name on there was ChippyTheLittleBrony. Site is still active. wayyy less active than it used to be though. There isn't much of an active community anymore since almost everyone, including PPA, have understandable lost interest.
As Chippy said, the site is still technically active, with people still posting relatively frequently, and posts are still accepted, but as for an active community, that's where it falls, there are a few people who are still active, but as a whole, there isn't much. And yeah, a lot of people have lost interest in it.
I myself still have a little bit of interest, but without the active community, I'm not so sure. I was working on a design a while ago, but have kinda lost interest in working on it. I'm considering submitting all the files for it in the designs forum here, so if anyone would want to finish it properly, they could.
One big issue the site is facing currently is that for a design to be approved, it needs around 10 upvotes, but even the best designs nowadays struggle to get 5, so any designs that get approved now are really old designs that just barely make it to those 10 upvotes.
A general theory I have is that what happened is people have started leaving, and as such, designs lose upvotes, meaning new designs never get approved, and as such, people get discouraged and leave the site themselves.
Something else I can see happening is the simple fact that once you've made something once or twice, the enjoyment goes away. In the case of Minecraft style papercraft, its pretty much always making cubes and cuboids, which can eventually start getting pretty boring.
I could go on for ages about the site, and for a while, I was even considering making a proper post and/or video about the state the site is in, how it fell, and how it can be brought back up. Whether I will or not is still uncertain. But to finish, I do remember you, I'm pretty sure by the time I joined you had left, so I doubt you would remember me.