Congratulations! You found the "loophole!"
For some silly reason, the NEC requires nearly every sort of abandoned to be removed, or capped off and identified at both ends ..... except power wiring.
Don't misunderstand me, we're not talking about an exception that mentions power wiring; we're saying that the NEC simply fails to mention this little detail in the "older" parts of the code.
Blame it on the educational system. When the part about power wiring was written, it was probably assumed that any moron could figure out that you didn't want live wires dangling in the ceiling. Later, when the NEC began to address 'low voltage' trades, standards had changed, and it was realized that it was, in fact, necessary to spell this out. Alas, the never want back and added this to the power wiring sections.
Panel ID is a wonderful thing - I've gone so far as to mark panels in Chinese as well as English - but we all know the limitations of panel ID.
There's probably no law or code saying you can't hang a sign upside down .... but, as with dangling hot wires, I wouldn't think very well of someone who did that.
(You think I jest? I have a customer, a mini-mart, who found several signs mounted upside down when he bought the place. It seems the former owner had literacy issues!