This is from the Google product support forums:
In general, a mutual subscription agreement between to users isn't disallowed. But, for the mere purpose of just gaining subscribers for a useless reason is very much frowned upon by YouTube and is in large part the reason why they implemented a 2000 subscription limit, trying to curtail this practice of sub4sub and moreover the spam that goes along with it. You really only want real legit subscribers that are actually going to be interested in your content and watching your videos.
It's possible to get your account banned if you're spamming it. If you are going to affiliated sub4sub channels and lobbying for subs via the sub4sub method, then probably not. But, if you're spamming sub4sub on channels that have nothing to do with this practice and doing it in a very unsolicited fashion, then it's possible.
Credit:
https://productforums.google.com/forum/m/#!topic/youtube/8yATG6z2_z0