There is much discussion, study, and misinformation in the Pokémon GO community regarding the Shiny rates of Pokémon. Some trainers incorrectly believe that catching more Pokémon instead of employing the Shiny check method will yield better results, while others see a certain location as a "lucky spot" for Shiny Pokémon. One of the persistent questions that players have is if Pokémon in raids have a higher Shiny rate than Pokémon in the wild. Let's investigate.
The simple answer is "no," but there's more to it than that. There are Egg/Raid-exclusive Pokémon with a higher shiny rate, but we will explain that shortly. First, the important thing when understanding Shiny rates is that the manner in which a Pokémon is encountered has no impact on its Shiny rate. These rates are set. Whether you see a Sandshrew in the wild or in a Tier One raid, it will have a Shiny rate of one in 450.
Now, there are certain Pokémon that has a boosted rate, but that rate is, also, regardless of the manner in which they are encountered. These rates are set by Niantic to make certain species more possible to hunt. Some of these species with boosted Shiny rates include Pokémon such as Sneasel, Scyther, Onix, Bronzor, Clamperl, Alolan Exeggutor, Feebas, Gligar, and more. These are what Silph Road researchers, those who have worked to determine these rates, refer to as "permaboosted." Regardless of encounter, though, these species will have the same rate if they are found in Raids, Eggs, or hatched. There are also Raid/Egg-only species, such as Shinx, Klink, Timburr, and more that have a similarly increased rate. Legendary Pokémon have their own set rate at approximately one in twenty.
Then, there are events that will boost a species' shiny rate for the duration of the event. Raid Day Pokémon are often boosted to one in approximately ten during their events, and earlier years saw many Pokémon with a boosted Shiny rate during their initial release, including Shuckle, Diglett, Yanma, and more. These Pokémon have since reverted to the standard rate.
In short, if the next new Shiny release is both in the raids and the wild, think for a moment before spending that raid pass. If that Pokémon fits the standard rate in the wild, it will feature the exact same rate if you put down a raid pass.