Sounds like a similar solution to gentoo's slotted ebuilds [0]. Switching the 'default' implementation system wide or per user (eselect) has the same issues discussed. Not sure if there is an existing utility (beyond your standard shell script) that can produce sane per-shell environments.
Mhm, doesn't sound all that similar to me. This looks like something that needs to be maintained on a per-package basis as a fairly ad-hoc solution for different library versions, in Nix the whole thing just falls out of the way it's set up in general.
0. https://devmanual.gentoo.org/general-concepts/slotting/index...