I've seen a couple videos on the Internet about legendaries and Ultra Beasts being coded to be not a shiny unless if someone is using ROM hacks like for example, Solgaleo and Lunala not being a shiny in the games due to code unless hacked. Since Xurkitree is my favorite Ultra Beast, has Xurkitree been coded not to be a shiny with hacking in Pokémon Sun and Moon? I just wanted to ask this so I wouldn't have to waste 30,000 soft resets on trying to find shiny Xurkitree. Thanks!
Yes, Xurkitree is shiny locked, which means you cannot get it's shiny form.
Source: http://www.serebii.net/games/shiny.shtml
Yes, along with the other legendaries, UBs and Tapus.
A full list of shiny-locked Pokémon can be found here.