(Update 4/18/21: Should be possible to add back your card now.)
Many people report getting an email from American Express that their card was removed from Google Pay for no apparent reason:
Your American Express® Card ending in _____ was removed from Google Pay on your phone. If your American Express Card account has not been cancelled, you can re-add your Card to Google Pay at any time.
Seems to be some sort of glitch – if you got notification of your card being removed, it’s not necessarily a sign your card is breached (or, at a minimum, it’s something happening to many people).
