Extend, Extended, Retract, Operate.......ALL PROHIBITED above 20000 feet for the reasons given in excellent link provided by miaguppy. It's a Mach Number limitation.
The 20000 feet quoted is a convenient rounded figure, perhaps you had a lucky day hetfield, you may not have encountered a significant Mach Number until, say, 23000 feet?
Perhaps someone can retreive the details of the B727 crew back in the 70's, who inadvertantly (that's being kind, there was fiddling involved) deployed the leading edge devices at high altitude (Low CAS, but high Mach Number). The almost total loss of control and severe structural damage suffered in the extremely high speed descent almost lead to the total loss of aircraft and it's occupants.
Regards,
Old Smokey