Was the usage "Spaghetti were" ever acceptable or common?

Solution 1:

In English it's usually an uncountable (mass) noun, with "piece of spaghetti" or "strand of spaghetti" used for the "singular", but in Italian spaghetti is plural (of "spaghetto"), and they freely use it as such. (See this website.) All the citations in the OED for "spaghetti" are consistent with its being used as a non-count noun, so I don't think Maugham's "his spaghetti were…" was ever normal usage.

Solution 2:

Maugham doesn't use it as an English word. In the book, the sentence is:

Dirk Stroeve flattered himself on his skill in cooking Italian dishes, and I confess that his spaghetti were very much better than his pictures.

Those italics are from the original. As such it's marked as being a foreign word used in English, rather than the English loan-word that came from it. It's worth noting that Italian cuisine wasn't very well-known in Britain until the 1960s (and it was still some time before those in Britain and Ireland who weren't Italian themselves went from boiling the stuff to a mush to thinking al dente means "removes teeth" before finally getting it half-way right), so using it as a foreign word rather than an English word wouldn't have been unusual for a British writer in 1919.

While as an English loan-word from Italian it's normally used in an uncountable sense, and so singular in grammatical number, it's a plural in the original Italian and hence when Maugham used it as a foreign borrowing it was appropriate to use it in the plural.