Display Authorisations for aerobatics will generally have at least two separate height minima, one for figures deemed to be aerobatic, and a lower one for a fly-by. The set of aerobatic figures which can be performed under a specific DA conform roughly to the different levels of competition aerobatics.
There are very few people around with a zero-base-height Unlimited aerobatic DA.
The barrel-roll has probably killed more display pilots than all other aerobatic figures put together - I'd take your reference to "50ft" with a large dose of salt.