Yes, vendor extensions are technically invalid but by no means does that mean you should remove them.
The validator only validates CSS that it knows. Those vendor extensions you just posted are browser-specific. All vendor extensions are technically invalid in the validators eyes. However, not all errors are actually errors that you should bother fixing.
I recommend removing them though (contradicting what I just said about vendor extensions) since box-shadow support is pretty good and goes back to IE9/FF33 etc.