To elaborate, people figured out how raid dens worked. Essentially, it's not randomly generating pokemon. There's a den "seed" that determines every characteristic of the pokemon for each day, including if it's shiny. People figured out how to get these seeds, and how to get the information from them. Now, there's a way for people to find out where the shiny frame (read: day) are whenever they use a wishing piece, and so using the date skip methods, they can get to a day where they are guaranteed to find a shiny pokemon.
But it doesn't end there! Apparently, the game locks in the species of pokemon for each den on the day you save, plus the next 2 days. However, 4 days out the pokemon species isn't determined. So, by saving 3 days out from a shiny frame, people can use the date advance to get a randomly generated pokemon from that den that will be shiny 100% of the time. You can look now and find people hosting shiny dens. It's easier than ever before to get shiny pokemon, and on top of that most of these are 4 IV or better. It's kind of hilarious how this has all gone down, but it's kept the game alive.