I have heard that cultures have called plant milks "milk" (Ex. Soy milk, almond milk) for thousands of years. Is this true? I would like a source to refer to.
I don't know about thousands of years, but in the Middle Ages in Europe almond milk was used as a substitute for milk on religious fast days, such as Fridays and during Lent, when drinking real milk was forbidden, and recipe books of the time did use the name "almond milk".
XLI. For to make Rosee.
Tak the flowris of Rosys and wasch hem wel in water and after bray hem wel in a morter and than tak Almondys and temper hem and seth hem and after tak flesch of capons or of hennys and hac yt smale and than bray hem wel in a morter and than do yt in the Rose so that the flesch acorde wyth the mylk and so that the mete be charchaunt and after do yt to the fyre to boyle and do thereto sugur and safroun that yt be wel ycolowrd and rosy of levys and of the forseyde flowrys and serve yt forth.
- The Forme of Cury, c. 1390.
(Quoted from http://www.foodsofengland.co.uk/book1390cury.htm ).
Almonds were themselves an expensive item, so you could say that this wasn't entirely in the spirit of a day of fasting. But that was mediaeval aristocrats for you.