Do we have specific examples of where a OW airline has refused to put an A or D passenger into an economy seat because there was no L available?
The above rules deal with whether A, D or L are "offered". That is different from "available". All it says is if the you have A or D and that's not offered on that flight, you go down - except on AA domestic where you can go up from D to A.
If I'm on a DONE4 and the flight I want is D0, any OW airline that would refuse to put me in an available economy seat has clearly lost the plot and has no clue what customer service is all about. They may well do it (and I'd be interested in anyone's specific examples where it has happened to them) but I would still complain loudly and vociferously if it ever happened to me.
I had a DONE5 booking held up where JNB-LHR was D0 and L0 and I was unable to confirm in either. I ended up rerouting to avoid Africa since waitlist showed no sign of clearing.
Other than in irrops (and you already have a confirmed booking), you have to have availability in the designated booking class.
This is one reason *A RTWs are better for economy (IMO) as they use a fairly high booking class.