Perhaps true for Canada but not in the USA. In the USA any cylinder filled with greater than 23.5% O2 must be O2 clean per CGA G4.1 as incorporated in CFR 49 171.7.
This is incorrect. If you read CFR 49 171.7 it incorporates CGA G4.1 only as it applies to CFR 49 178.338-15. 178.338-15 relates to the required cleaning for a cargo tank which is defined in the CFRs as:
Cargo tank means a bulk packaging that:
(1) Is a tank intended primarily for the carriage of liquids, gases, solids, or semi-solids and includes appurtenances, reinforcements, fittings, and closures (for tank, see §§ 178.337-1, 178.338-1, or 178.345-1, as applicable);
(2) Is permanently attached to or forms a part of a motor vehicle, or is not permanently attached to a motor vehicle but that, by reason of its size, construction, or attachment to a motor vehicle, is loaded or unloaded without being removed from the motor vehicle; and
(3) Is not fabricated under a specification for cylinders, intermediate bulk containers, multi-unit tank car tanks, portable tanks, or tank cars.
since a scuba tank is not permanently attached to a motor vehicle and is fabricated under a specification for cylinders it is not covered by the CFR.