Riroburo, a Pixiv user who has made the majority of his content paid, has been getting some flak on twitter recently and some of his paid art has been leaked. Are there any rules against posting leaked paid art or is it fine to use once made publicly available

If an artist asks us to take down thier art we will. We don't have the resources to keep everything tagged let alone keep track of what policies individual artists have on their stuff.
We're a small site, and for the most part not the first point of leakage anyway.

So don't worry about it. If an artist wants their stuff taken down they can ask. We've done this for the few who've asked so far.