People might be able to end or begin 2020 with a Pokemon Sword and Shield shiny Magikarp. A new Wild Area Max Raid event has begun. Until January 3, 2020, people have a better chance of encountering Magikarps in Pokemon Dens in the Wild Area.
As mentioned, the official website confirms this event will have increased shiny Pokemon odds. I can attest to the possibility, as the first Pokemon Sword and Shield Magikarp Max Raid I entered involved a shiny Magikarp with the new sparkling squares animation. (There are two shiny animations in this installment.) I also found that defeating the Magikarp gives you more Nugget and Big Nugget items than usual.
This is one of many special Max Raids that have already been scheduled for Pokemon Sword and Shield. When the game first launched, people had an easier time finding Gigantamax Butterfrees. Then, Gigantamax Snorlax showed up. Most recently, there were Delibird Max Raids appearing more often around Christmas 2019.
Pokemon Sword and Shield is available for the Nintendo Switch.