Last Friday new product M5StampS3 BAT (8MB PSRAM) shows where the M5Stamp controller will continue. It is a new generation that does not allow an increment from the original M5StampS3 or M5StampS3A version. It introduces a 24pins BTB connector and has directly 2x9@2.54mm pins with 11 GPIO.
I note that BTB nor 2x9pins is not a standard and neither an increment of any standard or a derivative of any standard, it is not even possible to use Atom (4 + 5 pins) modules, or AtomS3R BTB connector.
So, this new controller is not capable of stacking existing boards or other popular boards such as XIAO Seeed Studio.
So, for assembly compatibility, a piggy-back board needs to be designed, which would be inserted instead of the M5StampS3A and the new M5StampS3 BAT module would be inserted into it.