The GitHub contribution graph can be deceptive. The stuff Sylvan is working on these days tends to be deeper things that take a long time to get correct and often is a small amount of code.

He definitely isn't churning out tons of code for Pony these days. He is still, however, actively involved. From time to time, big new things from him show up. The latest one has been cooking for several months now and probably won't show up for a few more.