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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.
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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.
I think Dombrowski finished on the Leadville podium so it's clear some riders DO ride these events but I guess the UCI chooses to not enforce that rule. I would assume it's to stop the teams creating or competing in a breakaway series whilst continuing with the world tour.