Tim, I asked your question on the Military thread and the answer was
That is the yaw sensor for the auto stabilization system for the BN. Without auto stabilization, it was flyable but barely.
In Steve Davies "Red Eagles" all the USAF pilots hated the Mig-23BN and thought it was trying to kill them. That may be the result of the auto stabilization not working to spec!
Even with the stabilization on, at high IAS it would tend to dutch roll, but could go to a very high speed at low level (I remember 800 - 1000 kts being bandied about) while banging your helmet on the canopy from side to side. It could also sustain 7G in a turn all day long!
The Mig-23BN was a bastard child of the Mig-23MS and the Mig-27 - and those didn't have the same issues (just different ones).
You'll also find the yaw vanes on the SAAB Gripen, SU-25 and Mig-21 (not an exhaustive list, just ones I know of).