I notice that it is actually quite easy to delete anki cards accidentally in many contexts. For example pressing U can often delete the cards as well as accidental keystrokes in the browse tab.
Is there any setting that makes it harder to delete cards?
I’m not familiar with the U key being a shortcut at all in Anki. Which version of Anki are you using? Or is it maybe an add-on that is defining this key for you?
On the desktop Ctrl+Del deletes a note, but you can’t really delete an individual card with any shortcut. In the card editing screens you can do Remove Card Type to remove a particular card for every instance of some note type. Or you could edit the front template in such a way that it becomes blank for some cards, but even then you would need to run Empty Cards to actually make those blank front end cards go away.
Maybe try the Customize Keyboard Shortcuts add-on. I used it to disable almost every shortcut. I think the one you want to disable is "reviewer delete note". Mine says "reviewer delete note": "<nop>" and I assume it’s working because I haven’t accidentally been deleting cards.
(Now I just wish there were still a warning to prevent accidentially deleting entire decks!)