“datum” is the singular, but you’re right that it’s not really grammatically plural since you’d say “the data was stolen” not “the data were stolen”. I think the latter would technically also be valid; my interpretation would be that the latter is “countable” plural, so there are specific discrete datums that were stolen.
Confusingly, British English actually does treat nouns like “data” and “government” as plural where American English does not. Even more confusingly, they’re a little inconsistent with it, so you can find published examples of both.
“datum” is the singular, but you’re right that it’s not really grammatically plural since you’d say “the data was stolen” not “the data were stolen”. I think the latter would technically also be valid; my interpretation would be that the latter is “countable” plural, so there are specific discrete datums that were stolen.
Confusingly, British English actually does treat nouns like “data” and “government” as plural where American English does not. Even more confusingly, they’re a little inconsistent with it, so you can find published examples of both.