As of today, turbo is allowed for the B button (the firing button.) Inspired by the allowance of turbo in Metroid Dread speedruns, this is for the sake of accessibility, and preventing repetitive stress injuries.

For console:

• Use a third-party Gamecube controller that has turbo, to play on the Game Boy Player add-on for the Gamecube.

For emulator (Bizhawk):

• Config > Controllers > Clear the binding for the B button under the Normal Controls tab, and bind it to your preferred key under Autofire Controls.