I started a post on this same issue last November. If you tap on a link in a Mail message to launch Safari to that site when you return to mail you are unable to use the delete key on the keyboard, Magic Keyboard for me, to delete the message in the list. You have to go to another message in the list and then come back and then the delete key will work again. This occurred in 14.2 and was resolved in a later version. Now it’s back. I have access to Feedback Assistant for beta testers and have submitted a bug report. The original thread was :
forums.macrumors.com
ios 14.2 Disables Delete Key in Mail On Smart Folio and Magic Keyboard
You can delete text with the key but you cannot delete a message in the message list.