im::Vector :: tail/init has tail/init, where

tail = stuff except for front
init = stuff except for back

This appears to have been removed from the latest:

Question: Was this renamed? If it was removed, was there a rationale ?

The changelog says it was removed in version 11.0 (July 2018). The implementation changed in the same release. If you want the rationale beyond that, I'd contact the maintainer(s).