It's sideslip vane. I suppose you've seen it on BM, strike version, as interceptor versions have it on the upper portion of the nose. It can't be used to determine crab angle, as crab angle is relative to ground and even as you're crabbing, you're probably flying straight through airmass and your sideslip angle is likely to be zero.
Sorry, I can't help with what was its exact purpose on MiG-23 i.e. which system used it.
AFAIK, apart from slip ball there is no other sideslip indication in Flogger's cockpit. Hopefully, some MiG 23 pilot will shed some light on it.