|In the last 4 years, give or take, there has been a lot of discussion and work relating to adding features to old or sub-par web browsers.|
These have been called, variously, “polyfills,” “shims,” and “shivs.”
What’s the best term?
Well, a shiv is a homemade knife-like implement, such as one created and used in a prison environment. Where’s the word come from?
> “a razor,” 1915, variant of chive, thieves’ cant word for "knife" (1670s), of unknown origin. 
A “shim” is something that raises something up a level — that is exactly what we are doing within an old browser! Origin:
> “a slip of wood,” 1723, a Kentish word, of unknown origin. Originally a piece of iron fitted to a plow for scraping soil; meaning “thin slip to fill up a space or raise a level” is from 1860. 
Similarly, “polyfill” originates from “Polyfilla,” a brand of wall filler (or spackling paste, which is less relevant). That also kind of makes sense: it’s filling a gap (in a wall, and now in provided functionality).
Please — two terms is already too many and hurts searchability. Three is absurd. Let’s start with, ahem, cutting out the obvious worst option. Don’t say “shiv” when you mean “shim”!