To be honest, YouTube has an automated system that removes subscriptions that either have not checked out your channel at all, watching any of your videos, or participated in any comments on your videos. I personally believe this is a way of YouTube trying to remove the inconstancies of SUB4SUB and the idealisms of fake subscriptions by creating multiple channels under one name and subscribing to the main channel. Perhaps they're also removing channels that just don't use YouTube anymore.
A lot of people have moved to different platforms to record their videos, and I personally believe once streaming becomes a very stable and more reliable platform than recording a video and posting it, YouTube will probably be a thing of the past, it's really only a matter of time. But, in case it dose not.. I would highly suggest anyone who is doing SUB4SUB or asking for Botted Subs to stop, because Google is now on the look out.