The Cannes Film Festival has an unofficial de facto rule that women must wear heels on the red carpet. It’s quite a longstanding rule that has received backlash from female attendees, most recently from Cate Blanchett and Jennifer Lawrence.
One of the first stars to make a statement against the heel rule was Julia Roberts and Sasha Lane, who both went barefoot down the red carpet in 2016. Similarly, Kristen Stewart took off her Christian Louboutin pumps in the middle of the red carpet in 2018.

There is a dress code for Cannes as explicitly stated on the invitations, but it says nothing about high heels.
Chloë Sevigny told Vogue in 2021 that, “Being in France, there’s a level of elegance that’s encouraged – expected, maybe. But it is kind of old-fashioned and archaic rule.”
If there is no written rule about it, why are people getting mad? In 2015, a group of women in their 50s were turned away from the premiere of Carol because they were wearing flats. As it turned out, at least some of these women had medical conditions that prevented them from wearing heels. This story isn’t the only of its kind, but it was the one that sparked Roberts to ditch the heels the following year.

Blanchett’s choice not to wear shoes at all was in direct opposition to this rule though she didn’t do it on the red carpet, while Jennifer Lawrence rocked up in red heels that were shortly replaced by black thongs (flip flops for our American readers).

Fashion itself is moving away from the heels only rule as it’s beginning to feel outdated. Designers themselves have been providing more options like Dior’s Fall 2021 show which paired ball gowns with mesh flats, or Louis Vuitton pushing combat boots.
It seems that the unspoken rule may be phased out in the very near future.