Bellerophon

One possible reason you may not have seen it on other aircraft types is that I think it would depend on the type of emergency passenger oxygen system fitted to the aircraft; a bottled-oxygen system or a chemical-generator oxygen system.

Aircraft, such as the DC-10 which BA used to operate, had an emergency passenger oxygen supplied by a chemical-generators, which, once activated, could not be turned off, and would run until depletion. On those aircraft types cabin crew would supply therapeutic oxygen to an ailing passenger using small portable oxygen bottles.

To supply therapeutic oxygen, via a plug -in mask, to an individual passenger who needs it, requires the aircraft to have a ring-main passenger oxygen system, supplied and pressurised by oxygen bottles in the holds, with the associated ability to turn the oxygen supply off once therapeutic oxygen is no longer needed. In both cases, as mentioned by sammyh25 different flow rates can be selected by a switch on the mask.

As an aside, the procedure for turning on, and particularly off, the therapeutic oxygen on earlier series of B747, if not followed exactly, or if the passenger disconnected the mask themself, would result in all the passenger emergency oxygen masks dropping down, something I suspect some of the older more senior and experienced cabin crew on this forum may have experienced!