Did Ali make a habit out of holding...

No defending the behind the head holding, aside from that alot of what Ali was doing was leverage blocks and stop blocks. When a fighter does this it appears the fighter is atteempting to clinch or hold but is not.
The fighter is actually trying to detour his opponents punch with a leverage block.
With the stop block Ali was attempting to stop the punch before it launched. Both look decieving though as if he was trying to hold.
Foreman was another fighter who was good at these (legal) tactics.