NOTATIO

A question: is there some standard as to whether to indicate whether instruments are "plural" or not?

for example, would one add an "s" to the end of "violin" for a section? how about to the abbreviation?

I've looked through my orchestral scores, and in various languages, some have plurals, others don't.

I have two scores with instrument names in Italian, one with singular names and abbreviations, the other with plurals.
One English score with plurals, and another with singular.

Is it really just arbitrary? whatever you want? or are there historic periods where one was more prevalent than the other?

I suspect it's just down to house style.

The early 18th-century Italian sources I deal with tend to use plurals or singulars accurately, e.g. Violini, Bassi, Oboi; but Violino solo, Flauto, Tromba, etc.
Interestingly, voices are always singular -- Canto, Alto, etc, though that doesn't necessarily mean one to a part.

The singular form for sections is a kind of adjectival shorthand, where "Violin 1" means "the Violin 1 part/line", regardless of how many players there might be. It's perhaps a more conceptual organization.

But I doubt you'll find any hard and fast rules; and no one will be confused either way (which is the main consideration).

If I'm not wrong, the only indication that has to be plural is Bassi in older scores, for violoncelli and contrabbassi sharing the same notes on the same staff.