i don't think it will ever happen, the chance is just too small that somebody takes the burden to code this, which is imho not a matter of a just few code functions
if you leave one part of the map to access another the game would have to make a file like a safegame to keep status over killed npcs, mission targets, items, destroyed brushwork (breakables), active, passive and dead triggers, etc.
Halflife supported the system cuz it made a lot of variety in models, textures, sounds, scripts possible.
They used map sequences or rows, call it how you like it. If you safe while being in level 3 and you can still reaccess lvl 2 and 1, the safegame will be pretty big. But from time to time you passed points were a return was not possible anymore, so the size of the safegame was much smaller than before.
But i dunno if the realtime status of level sequences was cached or safed in a temporary file if you reached another level.