I don’t really know. I don’t think anyone knows, really.
He changed his Tumblr url to “horseness,” gave it a completely blank theme and has since deleted all its posts; and it seems he’s deleted most of his videos from YouTube, leaving only the unlisted Standard Model videos which are not connected to any public playlist on his channel. And it’s been all like that for the better part of a year.
SGaP has been known to be quiet and modest to a fault, so for all we know he could turn around at any time and drop a new project, much to the delight of his fans. Then again, it could be just as likely that he’s stopped producing music—at least for the time being—or has decided to leave the mlp fandom. I think none of these are particularly bad possibilities—at least due to his quiet nature, if he did in fact “leave the fandom” then he was able to escape the drama that befalls other former fandom contributors.
Whatever the case, what comes of SoGreatandPowerful is his own decision, and we should respect that. I don’t believe he was ever very pleased with his own work, and was constantly trying to outdo himself, which was no doubt taxing. If he plans on coming back, he’ll do it in his own time, and no amount of lauding him will coax him out.
In the meantime, remember that fans of his have made sure you can still hear all his music at fansite sogreatandpowerful.com, and watch his lovely PMVs on YouTube channel SoGreatandPowerfulArchives.
Oh—and do check out the #SoGreatandPowerful tag for fanart and other neat things!