Just reviving this as this issue has still not been fixed. There is a post here Not seeing sidechain input after enabling - Virtual Effects and Add-ons - Steinberg Forums. Putting a gate on a send channel to modulate a delay resulted in NO side chain inputs visible in the send menu (and some lost hair).
Putting a compressor in its place and activating the side chain, verifying its presence and then switching back to a gate fixed it. The side chain option was now visible in the Send. Frustrating, and seems to be something that can be both easily reproduced and fixed. Well, at least after the passing of 2 years.