This is a unique ID for a keyboard for the time it is connected to the system, and is never reused for the lifetime of the application.
Defined in <SDL3/SDL_keyboard.h>
typedef Uint32 SDL_KeyboardID;
If the keyboard is disconnected and reconnected, it will get a new ID.
The value 0 is an invalid ID.
This datatype is available since SDL 3.1.3.