While Meltan was available only for a limited time, there is still a way for Pokémon GO players to catch one. To get a Meltan, players require a Nintendo Account linked with Pokémon GO and a Pokémon Home account. Players can also use this method to capture a Shiny Meltan; however, that is only possible during special events.
Only a handful of Pokemon has their Shiny form removed after certain events. Normally, Shiny Pokemon are here to stay as part of the game's permanent roster. In the case of Meltan and Melmetal, its Shiny form is only added when something special is happening in the game. In this case, players will be able to find a Shiny Meltan as part of the game's 5th anniversary.
Meltan has been observed to have a rate of 1.6% (1 in 62.5; ×8) during the limited times when its Shiny form was available.
In conclusion, you can get shiny Meltan, but only during events.
According to here, the most recent event ended March 4th
Stay tuned for the next event in PoGo for your chance at a shiny. I believe Go Fest allows shiny Meltan, and it happens every summer.
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