It's not that the "-" prefix is just a harmless undocumented feature that only (a few) people who know about it use, it's more that it's an insidious undocumented foot gun that shoots (most) people who don't know about it when they use it accidentally.

Google should at least show a user friendly help message at the top of the page, that actually teaches users about the filtering feature, and lets you turn it off, like:

    Results containing "webkit-match-parent" omitted because of "-" prefix. (Click to include.)

But that would reduce the ad surface area by a tiny amount, so oh well...