That's a misunderstanding.

There was a short period when that was true on the 737 fleet but they quickly modified those airplanes so that it was not possible to turn it on. They are now (slowly) modifying those airplanes so that the Ch.9 audio feed comes directly from the #1 VHF communication radio, bypassing the observer's audio panel completely. The only way to tell the status of a particular airplane is to look at the OBS Audio switch, on the upper-overhead panel, to see if there is a small "INOP" placard next to the switch. If not, Ch.9 is active and available to use.

When I've flown an airplane with an active Ch.9 system I've found that a large percentage of the F/As are not even aware of the feature. I wouldn't count on them for information about Ch.9. It's better to talk directly to one of us up front. When you board, just as the F/A if you can say hello to the pilots.