Rabbonim do NOT monitor those who have been megayer and withdraw their geyrus if the gerim go OTD.
If they did, the ger would never be able to participate in communal Jewish life.
How could you ask a ger to join you for a zimmun — perhaps ten years from now he’ll go off the derech and have his geirus revoked.
Any shtar he signs on? Invalidated years later after the fact (and if the convert is a woman, then it would apply to her children, grandchildren and so on…). Hataras nedarim he might have helped you with? Well, now you’re stuck with your neder again.
How you could count him for a minyan? Perhaps years from now, your minyan will be retroactively nullified.
How could you rely on him to be a witness to your kiddushin, or worse, a get? Can you imagine all the nightmares it would cause if twenty years later a geirus could be revoked and all the gitten on which he served as a witness were invalidated years after the fact?
Or how about if the ger’s daughter married a Kohen? Not only would a revocation of his geirus end his marriage, but also his daughters (and render any grandkids as challalim).
So, yeah, it’s a good thing we don’t monitor geirim after the fact.
The Wolf