It’s just by coincidence that you never heard of the word “data” being used as a plural noun until after the pandemic, but that had nothing to do with it. I’ve long since rolled my eyes at this fact, especially when someone uses it to correct someone else. From the prescriptivist view, “data” is plural and “datum” is singular. This is how the word is prescribed. From the descriptivist view, the most common way people use the word “data” is as a singular form noun. “The data backs up my claim” sounds better than “The data back up my claim”, which sounds vaguely idiotic.
Caveat: unless one is writing an academic paper… in which case you’d damn sure better use the latter example, as the former will make you come across as an uneducated moron and this could bias your reader and/or a person grading your paper.
But no, it’s just one of those things you didn’t learn until later, one of those factoids that snuck by you for years. Personally, I didn’t put together the fact that Ecuador is called Ecuador because it sits on the equator until I was well into my 30s. Or, for example, there are a ton of words I simply don’t know how to pronounce bc I’ve only ever read them and never heard someone else say them. Some I’ve finally heard now, like “indefatigable”, “detritus”, “Basquiat”, “oeuvre”, “ancillary”, and “posthumously”, and then others I still don’t know how to pronounce exactly like “floccinaucinihilipilification”, “Kratom”, “Lophophoro williamsii”, “iboga” and “ibogaine” ¯\_(ツ)_/¯ Or I learn it and then forget it later…