It was announced by PokemonGoLive.com, that for the upcoming 25th Anniversary of Pokémon, an event to celebrate Kanto will be held. (As explained, the focus is on Kanto, because Kanto is the region that started it all)
By purchasing a paid ticket, players will eventually be able to capture a Shiny Mew. It is not known whether Niantic will hold an unpaid version of a Shiny Mew event in the future.
At this moment in time, while it is expected to be able to be transferred to HOME, there is no confirmation on the site regarding whether this Shiny Mew can be transferred to Pokémon HOME.
Source: https://pokemongolive.com/post/pokemongotour-kanto/?hl=en