I am playing on Pokemon Sun and I am close to finishing the game, I know that during the post-game I can go to the Lake of the Sunne and receive a free Cosmog. But I looked on https://m.bulbapedia.bulbagarden.net/wiki/Shiny_Pokemon and it said that it was not possible to get a legit shiny Cosmog, I just wanted to double check if anyone has managed it?
No. Cosmog is shiny locked in Pokemon Sun and Moon, and the only way to recieve it is by hacking. Bulbapedia is correct.