> Different past tense for singular vs plural? Are there any other words that have that besides "is"?

Non-archaic? Quite possibly not. English has done a very good job of shedding inflections.

But “span” (as a past tense of “spin” isn’t non-archaic).

> Wiktionary doesn't seem to mention it being singular, although I might be misunderstanding it:

My sources were citing discussion in the OED, but I haven't verified them.