This is about https://we.phorge.it/source/phorge/browse/master/src/applications/auth/engine/PhabricatorAuthSessionEngine.php

Copying from https://secure.phabricator.com/T13225 filed in 2018 which says

This should be removed after a reasonable period of time elapses (removing the compatibility code too quickly will log everyone out).

It's been six years and four months since https://we.phorge.it/rP1d34238dc94555466e15039ff6991b371ae294ef which says

In a few months we can throw away the old code. When we do, installs that skip upgrades for a long time may suffer a one-time logout, but I'll note this in the changelog.