Never having flown the ATR (nor any particular desire too either
), it was common procedure with two other types that I am aware of...the Lockheed Electra, and the ex-USAir Bac 1-11's.
Both types had no pax oxygen, but could be flown quite happily above FL250.
The BAC 1-11 of course had charts for higher altitudes, and Lockheed was pleased to provide same if asked.
Ferry is ferry, just that...no pax. Gee, what a surprise.
OTOH, there
are a few new guys around who seem to make up the rules as they go along, without understanding the possible consequences.