The following information has been extracted from disassembling a SGB1v2 firmware; it should be verified on other SGB revisions.
The SGB firmware explicitly ignores all commands with ID >= $1E. This leaves undocumented commands $19 to $1D inclusive.
Commands $1A to $1F (inclusive)’s handlers are stubs (only contain a
RTS). This is interesting, since the command-processing function
explicitly ignores commands $1E and $1F.
The game Donkey Kong (1994) appears to send this command, and it appears to set a flag in the SGB’s memory. It’s not known yet what it does, though.