I’m not familiar with the specific install/upgrade process on Gentoo so maybe I’m missing something, but what’s wrong with forcing new installations to use time64 and then forcing existing installs to do some of offline migration from a live disk a decade or so down the line? I feel like it’s probably somewhat uncommon for an installation of any distro to be used continuously for that amount of time (at least in a desktop context), and if anyone could be expected to be able to handle a manual intervention like this, it’s long-time Gentoo users.

The bonus of this would be that it wouldn’t be necessary to introduce a new lib* folder - the entire system either uses time64 or it doesn’t. Maybe this still wouldn’t be possible though depending on how source packages are distributed; like I said I dont really know Gentoo.