I have noticed that enabling airplane mode, it also doesn't disable bluetooth, as that is still green.
This does make a lot of sense though - especially when most people who are listening to music on newer devices will be using Bluetooth headphones.
I'd be surprised if many (if any) airlines still prohibited Bluetooth.
It actually did it for wifi too, so I couldn't turn off wifi, unless I went into the settings, as airplane mode check wifi on. However I think it could be a bug after upgrading ios. Because I have been fiddling with the toggles, and it does now seem to always turn it off when I toggle wifi off via the control centre, and that is without enabling airplane mode. What you have to make sure is that the wifi and bluetooth icons have the line though it. If it doesn't then, it is only disconnected from wifi, wifi isn't turned off completely.