While opinion is split on this, these days I don't think you can appeal to strictness alone to justify 'data' being plural, given the way usage has already gone

While opinion is split on this, these days I don't think you can appeal to strictness alone to justify 'data' being plural, given the way usage has already gone. People (like me) who say 'data is...' are simply using data as a mass noun (https://en.wikipedia.org/wiki/Mass_noun https://en.wikipedia.org/wiki/Mass_noun), which seems to be a reasonable direction to be taking the language. I may be influenced by the fact that I usually see 'datum' used to refer to a reference point or marker in some arbitrary space, and thus in my mind is now another word separate from 'data' (albeit with shared etymology). If I had more than one datum, I'd call them 'datums', though I've yet to test how acceptable this would be in a formal setting.