FWIW I have had a shufti at what used to be taught.
As stated in my own training manual Flight Briefing for Pilots Vol. I.(published 1961-9) written by the respected 'Birch & Bramson, circuit height is shown at 800 - 1000 ft.
Thus the "1000 ft unless otherwise stated" doesn't come from back then.
I agree it's often the case, but not the rule.
mikehallam