Everything is on-track for this goal, so we intend to drop support for template overrides in the week-commencing 2nd June.
Since this will happen before the 3.5 stable release, we have backported the admin warning banner to stable 3.4, so that communities on stable get a decent amount of warning before their update to 3.5.
(cc @RGJ, since I know you host many forums on stable)