There is NO CASA rule regarding the minimum altitude for stalling.
The only rule you'll find, will be in your company / club Ops Manual, if they state a standard that must be adopted by it's pilots / members.
These arbitary figures are handed down over generations and never questioned.
Here's another one:
It s**ts me to no end when a student is taught to conduct a 360 degree clearing turn at 30 degrees AoB, before then practicing a 45 degree AoB steep turn. As if conducting a 45 degrees AoB turn is somehow less safe and requires an entire orbit at slightly less AoB to mitigate risk.
A simple airspace clear procedure of looking out all around is sufficient, but people will just do what they were taught without thinking why.
And one more for good measure:
HASELL checks before steep turns. The Day VFR syllabus specifies that you should perform an 'airspace cleared' procedure before steep turns, not HASELL. But people do that one also, because that's what Instructors hand down between generations.