In my opinion it's not, but that's because of Mimic1 and not William Afton.

The reason I say that is because of the Princess Quest 4 ending (spoiler alert); After giving the Old Man your security mask, you are given a plush rabbit, the same plush that you can get in Help Wanted 1 (you have to do a sequence to stop Glitchtrap from body-swapping with you, and after a cutscene you find the plush in the main hub). Considering that Golden Freddy kept William's soul from moving on, trapping him in a nightmarish coma (Ultimate Custom Night, based on related elements in "The Man in Room 1280), William's agony may have been the only thing left on the Springtrap/Scraptrap CPU that Fazbear LLC recovered. When Mimic1 scanned the CPU, it somehow remembered what William's SpringBonnie costume looked like before the springlock system was implemented for Fazbear mascots.
Back to the plush rabbit, when we receive the plush and take the elevator to the giant claw machine, Vanny is watching us as we hand Moon the plush, who then drop the plush into the prize box for Vanny. Once Vanny has the original Glitchtrap in her palm, she crushes him into oblivion and disappears into the darkness. Since Glitchtrap was made using William's agony, which can be conducted by remnant as seen in Special Delivery's "Shadowy Remnant," destroying Glitchtrap severs William's tether to reality and leaves him to his demons in True Hell, not Ultimate Custom Night which is most likely just a waking nightmare made by the vengeful spirit of Golden Freddy.

In short, even though William Afton is gone for good this time, Glitchtrap still remains as his copycat, continuing his legacy and fulfilling his wish to make the world pay him his cruel pleasures.