In the United States, Part 121 operations can apparently can be conducted with cabin altitudes above 15,000 ft as long as all passengers are provided with supplemental oxygen when the cabin altitude is actually above 15,000 ft (source: 121.327 and 121.329). The requirements around supplemental oxygen for crew are stricter, but they don't really matter for this question.
For EASA, the rules for commercial operations seem to require supplemental oxygen for all passengers above 15,000 ft in pressurized aircraft and 13,000 ft in unpressurized aircraft (source: CAT.IDE.A.235 and CAT.IDE.A.240, please point out if these are not the correct sources as I'm unfamiliar with how EASA rules are structured).
However, I could not find an example of these rules being used to operate commercial flights at high altitudes in unpressurized aircraft, or in aircraft only pressurized to a very high cabin altitude.
Are there any commercial flights worldwide where, during normal operations, passengers need to use supplemental oxygen? If not, are such flights prohibited by some other rule(s)?