I use a SwitchEasy CoverBuddy Case with my 2018 12.9 iPad Pro alongside my Magic Keyboard, and have had no issues.
The third party cover adds 0.08" (~2mm) extra thickness to the iPad Pro, which has had no impact on using the Magic Keyboard. With the new 2021 12.9 iPad Pro *only* being 0.5mm thicker, you should be able to use the new iPad Pros with the original Magic Keyboards.
The third party cover adds 0.08" (~2mm) extra thickness to the iPad Pro, which has had no impact on using the Magic Keyboard. With the new 2021 12.9 iPad Pro *only* being 0.5mm thicker, you should be able to use the new iPad Pros with the original Magic Keyboards.