Development is still happening, no worries!
Things aren't going particularly fast, after all we're still just three guys working on wallhaven in our spare time. Sometimes real life demands we take a little break but so far we've always returned without fail.
If it seems like we've slowed down that's mostly because big new features are becoming rarer and most of our work centres around improving things behind the scenes. For example I recently migrated all of our JavaScript to ES6, which is a big relief for us but (hopefully) won't have any impact on users at all (it's not live yet).
Apart from that the next update will bring the ability to reply to profile comments. AksumkA has been working on the long anticipated duplicate detection, which is still in an early stage but already showing promising results.