For the 25th Anniversary of Pokémon, an event to celebrate Kanto was held.
By purchasing a paid ticket, players were able to capture a Shiny Mew. At the time of this post, there is no unpaid version of the Shiny Mew.
In order to obtain this Shiny Mew, players had to complete a Special Research line that was pretty tedious.
Some tasks included reaching trainer level 40 in Pokémon GO, catching a Pokémon 30 days in a row, walking 151 km, catching 1510 Pokémon, completing 151 research tasks.
Various details on this file has been changed to keep the provider's account details private.
Thanks to @Beyonder107 for providing a copy of this Mew, as well as the necessarily evidence for this Mew.
Original article and discussion: https://projectpokemon.org/home/forums/topic/58269-go-shiny-mew/