If they want to make a rule, just say that the bars can't be narrower then rider's shoulder width. Problem solved.
Or - if broad shouldered riders like Kruijswijk and Welsford are going to find that unfair - simple word it something like this:
"Handlebars cannot be narrower than 40 CM, or the rider's shoulder width - whichever is narrowest."
So a rider with 45 CM shoulder width can still go down to 40 CM handlebars, while a rider with 30 CM shoulder width can stay with that size handlebars.
(Unless, of course, it's a safety concern, and UCI is worried that too narrow handlebars will limit riders' control. In which case; surely too wide handlebars will also limit riders' control...)
Another guy weighed in:
The new UCI rulings might be well intentioned, but once again women’s cycling seems to have been overlooked
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