I was reading this documentation file of some software and note the plural spelling of this abbreviation is “URL’s”. Why isn’t it “URLs”?
The regular way to pluralize any noun is by adding just an s; the apostrophe should only be added to plural s if the word would otherwise become unreadable or exceedingly ambiguous. A good example would be s’s (the plural of the letter s: ss would look like an acronym).
The word URLs would seem to be quite clear: *URL’s is probably a simple error made by those confusing it with the possessive ‘s, which always has an apostrophe (a few pronouns being the exception, like its and hers).