alt Hacker NewsAll I can say is that Gentoo specifically forbids downgrading glibc. I think it's the only package where they did this. [1] The .ebuild is full of patches and workarounds.
[1] See line 873. https://github.com/gentoo/gentoo/blob/master/sys-libs/glibc/...
Yeah, that's forwards-compatibility, which is different. Something built targeting a newer version of glibc is unlikely to work with an older one. (and gentoo will by default build against the latest you have installed, so yeah, downgrading has a good chance of breaking near everything).