This doesn’t really have anything to do with Pokemon GO, at least not directly. Instead it seems to be more related to the upcoming launch of Pokemon Sun/Moon, which apparently is Nintendo’s most pre-sold game in the company’s history, so safe to say that many fans will no doubt want to take advantage of this.
The stickers are a bit limited at the moment and only seem to include Rowlet, Litten, Popplio, and a Poke Ball. We’re not sure if there are plans to eventually add more Pokemon to the list, but since the stickers are free, we guess we can’t really complain. Users who want to check out the stickers will be able to access them via Twitter’s photo editor in which you can then use the stickers and add them to your existing photos before you upload them.
Pokemon Sun/Moon is expected to be released on the 18th of November, 2016.