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Not sure whether this is strictly enforced and the status of some of the races we're talking about (obviously TCR isn't sanctioned) but from UCI rules:
No licence holder may participate in an event that has not been included on a national,
continental or world calendar or that has not been recognised by a national federation, a
continental confederation or the UCI.
atz-
Ah, well that's the UCI protecting their asses from splinter groups I guess. The rule goes on about exceptions for "events whose format is not covered by the UCI regulations." so stuff like TCR, SSWC, Crocodile Trophy, Leadville, and the epic Grindleford Goat etc.
I'm sure current pros (certainly doping banned ones) have raced in sportifs, which are equivalent in format, ie. road races.
hippy
Not UCI, if it was specified it'd be in individual rider's contracts with teams. Presumably because they don't want their pedaling meat stick to injure themselves in some race full of gumbys. Almost certainly if it is a common contract obligation it's ignored by teams... right up until something goes wrong and they can use it to terminate said contract.