According to iGeneration, the newly launched 12,9-inch iPad Pro is not compatible with the previous generation Magic Keyboard due to a slight increase in the thickness of the new iPad model.
Citing a document sent to Apple Stores, the report says the new iPad Pro is 0,5mm. thicker than the previous 12,9-inch model. Due to this increase in thickness, the new iPad will not work with the previous generation Magic Keyboard. Although an increase of 0,5 mm. It may seem negligible, it is enough that the new iPad Pro does not close properly when used with the previous generation of keyboard.
The new 11-inch iPad Pro supports the previous generation of Magic Keyboard.
Along with the new iPad Pro, Apple has also marketed a new compatible Magic Keyboard and, in addition to compatibility with the new and thicker iPad, has introduced a new blank option.
As you surely know, the new 11 and 12,9-inch iPad Pro include the new M1 processor, 5G connectivity, and the new Liquid Retina XDR screen in the 12,9-inch model (which, although it can be pre-ordered starting April 30, will not start shipping until the second half of May).
Updated on 30/4/21
Apple has posted a document stating (sadly it hasn't updated its Spanish version yet):
The 12,9st generation Magic Keyboard is compatible with the new 5-inch iPad Pro (XNUMXth generation) with Liquid Retina Display. Due to the slightly higher thickness of the new iPad Pro, the Magic Keyboard may not fit snugly when closed, especially if a screen protector has been applied
The first generation of the Magic Keyboard (A1998) is functionally compatible with the new 12,9-inch iPad Pro (5th generation) with Liquid Retina XDR display. Due to the slightly thicker size of this new iPad Pro, the Magic Keyboard may not fit snugly when closed, especially when screen protectors are applied.