Why not label the panel with the maximum possible voltage that it can produce? Or even have a little table showing the maximum possible voltage at various ambient temperatures. It's not as if there isn't room on the back of the panel for a big enough label.
That is in fact useful information to someone that needs that info, because it means they should never eat it. I think this is a bad example, because I have no idea what you're talking about.
This is information is generally stated on the module datasheets, which specify the Open Circuit Voltage (V_OC) at Standard Test Conditions (STC), and then provide a temperature coefficient for how that voltage changes with temperature.
'Maximum voltage' is very arbitrary as this is directly dependant on the lowest expected operating temperature, hence the industry has landed on stating these values at standardized conditions (STC and NOCT) allowing for direct comparison.
The label on the modules themselves tend to also provide these ratings at STC, e.g. this label from Jinko specifies the Open circuit voltage and also summarizes the conditions assumed for STC:
While I agree that the label could also add the temperature coefficient, I'm not sure if it's reasonable to expect that specialist electrical equipment details all of its operating parameters on an attached label without the expectation of consulting a datasheet or manual. For specific products that primarily target non-specialised consumers however, a different labelling approach may be warranted.
