With modern fillrates and deferred rendering, is occlussion culling still relevant?
Yes, occlusion culling is still worth it.

At minimum, a draw call that you skipped due to culling is a draw call that doesn't have to run the vertex shader. Triangle count goes up as quickly as GPUs start supporting more triangles, because why not? With unified architectures, vertex shaders use the exact same hardware that pixel shaders do, so every vertex that you skip because of culling is more compute time for the stuff that you can see. Not to mention all the other stuff you're skipping (CPU draw call processing, and throwing the tris far enough through the pipeline that the rasterizer realizes that it doesn't need to shade them).

There was a great presentation from two Ubisoft studios at SIGGRAPH 2015 about GPU-driven rendering pipelines. On the surface it's about some of the things you mentioned: batching and instancing, reducing draw call counts. But one of the major advantages they get out of a GPU-driven pipeline is incredibly fine-grained occlusion culling: better culling than you would normally see at the draw-call level. It's all in the service of asymptotically approaching the goal: processing only what you can see, which means that what you can see looks better.

(Also: consider console, mobile, VR, and desktop-without-the-latest-and-greatest-GPU-that-money-can-buy. Even if all your tris disappear into the gaping maw of your top-of-the-line GPU, you may not be the primary target.)