A Pokemon that is Shiny locked is set to never appear Shiny. Shiny locking was introduced in Gen 5 to stop Zekrom, Reshiram and Victini from appearing Shiny. However, it is a misconception that Shiny locks apply to entire species broadly — term is most accurately used to describe Pokemon that are set to be non-Shiny in a particular in-game encounter (e.g. some gifts and static encounters). For example, your starter Pokemon in Sword/Shield is Shiny locked, but you can get Shiny Grookey, Scorbunny and Sobble through other means, such as breeding.
Even if a Pokemon is only obtainable in-game in an instance where it is Shiny locked, its species can be obtained Shiny if its Shiny form is available through an event or by transfer. For example, the Poipole obtained in the Crown Tundra in Sword/Shield is Shiny locked, but you can transfer a Shiny Poipole from Gen 7 into Sword/Shield. Furthermore, the Xerneas and Yveltal encountered as part of X/Y's storyline are Shiny locked, but Shiny Xerneas and Yveltal were distributed by events in Gen 6.
Still, not every Pokemon has been made available Shiny. If a species is completely Shiny locked (or not available in-game to begin with) and there is no way to get its Shiny though non-traditional means (e.g. events and transfer), then its Shiny form is wholly unobtainable. Most mythical Pokemon released in Gen 5 onward are still unobtainable Shiny. This list on Bulbapedia will tell you which Pokemon are unobtainable Shiny, and in which generations.
The list at the very bottom of this Serebii page shows all the Pokemon that are Shiny locked, and where they are encountered. The same page also includes a list of Shiny Pokemon that have never been made available in any generation, which might be easier to understand than the generation-specific Bulbapedia list. (Most events are set to never be Shiny unless the event is specifically for the Shiny form.)
