Apple documentation is very clear as to the user interface devices supported by the iPadOS 13.4 update. The Apple Magic Keyboard, Magic Mouse 2 and Magic Trackpad 2 all have full support - plus generic support is added for third-party input devices.
The original Magic Mouse and Trackpad were not designed or intended to operate with iPadOS - and whilst not supported by iPadOS, remain fully operable with the Mac computers for which they were designed to operate.
Generic device support provides basic input functionality. For example, many Mice require a manufacturer-supplied “driver” to be installed to enable additional “buttons” (beyond the basic/generic left/right mouse-buttons) and any proprietary “touch” functionality. Additional device driver installation is not (and never has been) supported on iOS platforms. For this reason, basic devices, such as a “basic” mouse seem to have full OS support - whereas a feature-rich Mouse (with many bells-and-whistles) only provides its basic functions when connected to iPad.
Many here are clearly disappointed in that the older-generation Magic Mouse/Trackpad, expressly designed for Mac, are not supported by iPadOS - however, all computing systems have limits as to the type of devices that it can support. Older hardware and devices are often not forward-compatible with new operating systems.