Following promotional pictures were published by creators of the game across their social media channels.
During the Halloween event of 2017, there were several Shiny Pokémon released in parts:
On October 20th, 2017 - Shiny Sableye began appearing on the first day of the Halloween event of 2017.
On October 26th, 2017 - Shiny Duskull and its family Pokémon began to appear six days into the event. 
On October 31st, 2017 - Shiny Shuppet and its family Pokémon began to appear 11 days into the event.
During this event , there were shiny forms of Gastlyfamily members added to game data, although they weren't released to be caught in gameplay. 
Shiny variants of family members of both Gastly and Misdreavus families were highly anticipated by players to appear during Halloween event in 2017 and later 2018, however neither of these Pokémon's Shiny form were released then.
However, shiny variants of Gastly and its evolved forms were released on Gengar Day, and shiny forms of Misdreavus and Mismagius got released on December 1st, 2018 with new Field Research Tasks for that month.