They're not in the Scourge of the Sword Coast interim rules documents either.
Look, Wizards are pretty fine with people playing whatever they like for Encounters. There's no "banned" list. The Warforged and Kender appeared in the last of the playtest packets as examples of races you can use to customise worlds. They're both entirely inappropriate for the Forgotten Realms, but if you want to use them and your DM agrees? That's fine. Go ahead!
Personally, I find Warforged problematic. I think the immunity to breathing thing (and @
jodyjohnson , the notes for cloudkill say that constructs are not affected) causes problems which the playtest version doesn't adequately balance. That said, the lich should realise that you're immune and use another spell... like an 8d6 fireball!
This encounters season is particularly challenging to run for multiple groups, though. I'll write my description of what happened at my store soon. (17 players, 3 DMs and me as co-ordinator). Lots of judgement needed so the players are challenged but not overwhelmed. Occasionally, you'll get a group of players who waltz through encounters, but I can say that at our store we had 6 PCs unconscious and dying at some point during the evening...
Cheers!