An ADF may not be used in an airway, but it is easiest and most available means of getting from the airway to the airfield without RNAV or GPS
Only if the airfield has an NDB on it.
The other problem is that only a STAR and an IAP will give you "official" obstacle clearance from the airway to the runway. Otherwise, you are doing the "DIY descent" thing, which is OK but probably not wise with NDB navigation alone.
Also I don't think there are many STARs that terminate on an NDB; most seem to use a nearby VOR, even if the IAP itself does not use a VOR.
The other thing is that BRNAV is mandatory in Europe above about FL095 so one has to ask what you were doing in the airway (most practical European airways routes will take you above FL095) in the first place if you did not have RNAV capability.
I have found an ADF useful as a backup for GPS, on long VFR flights, in certain places (France comes to mind) where there is no VOR in range if routing OCAS. It's much more useful as a locator in approaches.
But one doesn't have to do much IFR (airways) flying to realise that without RNAV it is very very difficult to manage in today's airspace. The whole process is geared up on you being able to fly A - B - C - D where all these are virtual points in space. Ground based navaids are just about irrelevant now. The other day I lost BRNAV capability and was limited to VOR/DME nav. A huge spanner is dropped into the navigation process.... for all the navaid work one does in an IR it is rarely used.
But this is talking about practical things. Fuji's original Q was about the legality.