I recently had a vault effectively destroyed by Deathclaws in Survival mode. I have seen it suggested to prepare for Deathclaws by growing your vault to 35 to unlock Endurance training and then dismiss some dwellers to dip back below the Deathclaw threshold, but I know unlocked rooms remain unlocked if your population declines - do Deathclaws remain unlocked too?
No, what attacks your vault is only determined by current vault population. If you drop enough dwellers you will go back to getting attacked by just raiders and/or ghouls. :)