Information surrounding Pokemon Sword and Shield leaked ahead of its official November 15 release date, and social media is currently aflame with opinions and confusion over what's changed, and what hasn't. There's been some particularly heavy controversy about "catch limits" that disallow you from catching extremely high-level Pokemon in the Wild Area until you have a certain number of Gym badges. The inclusion of these limits have some fans convinced they might stumble on a shiny Pokemon only to be told their level isn't high enough to catch it. The catch limit is a little more complicated than that, however.
When you first arrive in the Galar Region's Wild Area, you quickly come across Pokemon who are ten or fifteen levels above you. These are specifically identified as "very strong Pokemon." You might manage to take these strong Pokemon down for lots of experience points if you have the right team, but you won't be able to catch them. Any attempt to throw a PokeBall informs you the Pokemon won't allow itself to be caught.
This catch limit exists for the same reason high-level Pokemon you receive from trades won't listen to you until you have enough Gym badges. Namely, Game Freak doesn't want you to sweep the early game with an Onix that's 15 levels higher than the surrounding wildlife. However, there's no need to fear coming across a super strong shiny Pokemon you can't catch: these "very strong Pokemon" aren't random spawns. They're anchored to a certain part of the Wild Area, and they don't seem to come in shiny varieties. Note that lower-level Pokemon spawn in the tall grass, whereas very strong Pokemon tend to hang out in the open.
The very strong Pokemon respawn quickly, so even if you take one down for the experience points, you can come back and catch them when you have the required number of badges. If you want to hunt for shiny Pokemon, just stick to the grass as per usual.
Pokemon Sword and Shield is out this Friday, and we have in-depth impressions of the single-player experience. It might take you some time to get used to the game's bigger changes, e.g. the loss and replacement of the Global Trade System.