I plan to go back to Nier once I'm done with Automata. I hope it doesn't spoil too much of the first game.
I'm not done with Automata yet (only just started route C), but so far the only thing I've seen that could be called a spoiler for the first Nier were a bunch of optional text documents I found near the end of route B. It's possible there's some more blatant stuff later on that I haven't seen yet, though.
Honestly, if you really care about remaining unspoiled you should probably just put Automata on hold and go back to the original now. Or do like me and just watch the Clemps videos. Like Grief said, it's a pretty in-depth recap and even goes into some of the stuff that's outside the game (drama CDs and a short story, IIRC). I found the humor in them to be a little grating but that's just me.