The term for such words is “substantive noun”, because they refer to substances. Other examples include metal, water, wood, dirt, and plastic. Words that can go either way include stone and rock (you can say “I have a rock”, or “I have a lot of rock”, and either could be correct).
Collective nouns are something else entirely. They’re usually singular in form, but plural in meaning, such as “flock” or “herd”. Some language gurus are now saying that it’s acceptable to use plural forms with collectives, such as “The flock of geese are flying south”, but personally, that just grates on my nerves-- What if you want to refer to two flocks?
I remember a Brittish comedy-variety show (though I don’t remember the show’s name) skit done in ancient Rome. A guy goes up to the bar and asks for a martinus. The bartender says, “Don’t you mean a ‘martini’?” The first guy yells, “If I’d wanted a double, I’d have asked for it!”
In the case of data, though, while it has an English singular, it’s so rarely used as to be almost a nonword. I first encountered the word data over 25 years ago (when I studied data processing in school) and in that time I don’t believe I’ve ever heard the word datum used as its singular or heard data used with a plural verb. I’ve seen it written with a plural verb many times, but this is the result of the house rules of various publishers.
Datum, by the way, is also a technical word used in surveying and mechanical engineering. It’s plural is datums.
And to contribute to the words without a singluar: folks.
Since Alphagene flagged “cattle” which was my first thought when I saw the thread title, I must pick on his choices…
While the list of things to be acted upon is the most common use for “agenda” the word still maintains a precarious usage as the collective “things to be acted upon” that are on the list, and as such is a plural with the singular agendum…“item on the agenda” in the more common meaning of the word.
I dunno. The dictionary says it’s singular, but I’m having a hard time thinking of a usage where folk refers to a single person. (And please, no one bring up “folk singer”, that’s an adjective.)