Do you mean the RDMA verbs API? Or something else? I'll point out that it's an API, so an implementation pattern isn't relevant. (it's also an awfully obscure one, and I say that as someone who was involved in standardizing the pre-Infiniband Virtual Interface Architecture API that it derives from)
It's a relic of the old 7-layer model, where you looked at each layer in isolation, and was ditched in high-performance networking code starting with Van Jacobsen's in maybe 1991. No one ever hit 10mbit/sec with a state machine-coded stack.
Note that very few people write network protocol implementations, so it baffles me why they thought to suggest that their high-level design pattern would be useful in low-level kernel code.
The state machine pattern is used currently in verbs. That was the state of the art for many decades before higher level interfaces appeared.
Why was it a bad idea? Can you elaborate?