Even though the dimensions of the new iPad Pros are identical to their predecessors, the new camera array and LiDAR sensor means you’ll ultimately need to pick up a case with a larger hole. This is similar to what happened with last year’s iPhone 11 lineup, where all three models retained the same physical dimensions of the iPhone XR and iPhone XS that came before, but required expanded cutouts for the larger cameras.
In terms of other accessories, the 2020 iPad Pro uses the same second-generation Apple Pencil as the 2018 model (and of course it’s still not compatible with the original first-gen version of the stylus), and charges it in the exact same way through a inductive magnetic connection on the edge of the iPad Pro frame.
The Smart Connector on the new iPad Pro remains in the same place as the 2018 iPad Pro, which moved it from the edge to the rear of the iPad. Apple’s Smart Keyboard Folio and other similar cases that make use of the Smart Connector should continue to work just fine with the new iPad Pro, but Smart Connector accessories for pre-2018 iPad Pro models will remain incompatible, as they rely on the older edge placement, now only found on the 2019 iPad Air and 10.2-inch iPad.