Sylveon is the latest evolution of Eevee (or, Eeveelution) that was released along with the Kalos region in Generation Six of the main Pokémon games. As a Fairy-type, Sylveon is both a very useful and fan-favorite Pokémon. There was speculation that Niantic would not begin to roll out Kalos Pokémon in 2020 at all, due to the amount of Generation Five species that have yet to be introduced into the game. However, Pokémon GO's official anniversary poster, which teases what's to come ever year, indeed showed the Generation Six starters. So with Niantic teasing Generation Six already, many trainers are wondering when Sylveon will be coming to Pokémon GO.
Niantic knows the value of Pokémon's biggest icons, and how to use such species to prolong interest in Pokémon GO. The Generation Six first wave will be a huge bump in interest among trainers, so it's almost certain that Sylveon, as a major fan-favorite of the Kalos region, will not be in the first wave. It is likely that, like Leafeon and Glaceon, Sylveon will be released a long while after the Kalos starters to coincide with an event framed around its release. Mid-2021 is the earliest we'd predict for a Sylveon release, but there is one positive here… with Eevee already released in its Shiny form, it is likely that trainers will be able to unlock Shiny Sylveon upon its release.
Interestingly, the main series of games saw Eevee's evolution into Sylveon tied to affection for and friendship with its trainer. With Pokémon GO's buddy system, Niantic already has a built-in way to implement this: Best Buddies. Though there has been no confirmation of this, clever trainers would do well to slowly level their favorite Eevee up to Best Buddies to prepare for the eventuality of Sylveon.