Not that it was changed compaired to Old CPC generation (yet few CPC games actually supported the 2 button feature, despite available).
And of course the Atari standard was originally a 1 button solution (Atari 8 bit 2600 consoles) but cxould could easily be improuved to more buttons.
Sega Megadrives managed easily 6-8 button (Street Fighter 2 standard).
But as a result it is theorically possible to customise some kind of 6+ buttons Joystick quite easily, if only softwares software would support this.
*Simplest way : using the 4 directions of the second Joystick (on a plus or using a Y-double connector/cable on a CPC) as additionnal Fire Buttons, easy to use in 1 player games : you then put one joypad in each hand (the "nunchaku" technic).
It seems the "Power" button on SMS controlers is used when the controler is fitted with AutoFire option (needs to be checked) or when using the Lightgun.
But an external adapter may include such power-supply feature too so may works well with some mouses or Lightguns if softwares software support thoses.
==Nunchakus technique==