So in other words, the error was *not* fixed/included in the most recent release (v.2.2.4 or 2.2.5)?
Correct. We have deployed a fix, but it will only be included in the next Jetpack release.
I was concerned it might be a new vulnerability, rather than just a coding error, since it's surprising it's so pervasive.
It actually only appears for sites using the WP_DEBUG constant. Since this constant is used for debugging only, I find it a bit surprising to see so many results on Google. But the problem will disappear from all these sites as soon as we release a new version of Jetpack.