This is because hackers were using this to lock people out of their own accounts. They would add a PGP key and then the user could no longer read any emails from FB to recover their account. There are maybe alternative solutions, but it's not a bad reason to remove it IMO.
If they can set the PGP key they can also change the email. If the account recovery team allows access to recently removed emails as part of the recovery process then it should also allow contacting those addresses without a recently added PGP key.
Logically adding a PGP key is equivalent to changing the email, the previous person can't access the messages anymore. If the recovery process handles these cases differently it is a flaw in the process.
