for one doesn't implement v1's get signon (0x31)
I'm pretty sure it's one of the commands that optiboot handles in it's default "else" case; all of which it answers with "yeah, sure" (STK_INSYNC), and if that's not appropriate it's up to the other side to complain, or send additional bytes which result in reboot... If not, there's "0 " (GET_SYNC), which we can see the bootloader responding to in a "very verbose" avrdude log:
avrdude: Send: 0 [30] [20]
avrdude: Recv: . [14]
and 0x31 is leave programming mode in v2, so not great as a test cmd.
Doesn't matter if it exits; you can just reset it again, and you know what's there...
Besides, in stk500v2, the "leave programming mode" would have to be wrapped in a message with a proper CRC and whatnot...