“I’ve never heard of a kiruv rabbi deterring a potential BT with regards to shidduchim”
Thats because his kiruv would probably not be effective if the person the rabbi was being mekarev found out he would never be good enough to join the Rabbis family.
If a potential convert will probably not find a shidduch in the community and suffer because of this, it is the Rabbis OBLIGATION to inform the potential convert of this possibility in ADVANCE.