IE8 seems to be the last browser that didn't provide this natively. It's a bit tricky to test because it needs a real event and not a simulated one.

#3233 (comment)

See also gh-2337, if we're not filling the property and just passing it on I don't know that we need to formally deprecate it in jQuery or generate a Migrate warning. If browsers ever tried to deprecate it via a console warning, the pass-through we do when copying the property would be the responsibility of the caller. If anything I'd say we add a note in our docs to use event.button or event.buttons where possible.