Cyker: The game probably still has hooks for LAN multiplayer, since the original versions all had it built in, they just went to the effort to remove it for reasons I don't understand. It might be possible for someone to recreate the DLLs to hook into it and patch the menu, but it'd be a lot of work and reverse engineering! Definitely not something that'd happen any time soon.
Is the removal of LAN something Steam requires or something? I just don't see why else they'd go to the effort of removing it. If they were beholden to another publisher like Sega or Bethesda or Ubisoft or EA I can imagine the removing of LAN being ordered by them, but if Relic own it lock, stock and barrel now, what's the reasoning...?
The primary reason is to make pirating harder or less rewarding. I doubt Steam itself would request something like this, I'm sure it was the studio's decision on a level.
I heard that they rebuilt the engine. This might be only a marketing slogen though, to make it sound like the game worth more than a simple remake. If the engine really got rebuilt then it is possible the LAN was completely cut out from the code. If the remake is more like a patch, then LAN should be there somewhere, functional at least on code level.