I know it was absolutely a thing in the past, my parents talked about it for example, but I've never seen all-gender changing rooms in a French swimming pool. Though I've seen the obvious signs of the retrofit in a few places, where you can clearly see that it used to be one big room that got separated in two.

I think you're right that they don't trust us to properly shower before going into the pool though, because to this day, I still see like ~40% of people barely showering enough to rinse their sweat off. A lot more people are actually showering than when I was a kid though (myself included)

Edit : actually, I've been in some pools that were constructed more recently that have mixed changing rooms, but these have individual stalls and the showers are still gendered

TLDR : the showers aren't all-genders as far as I know. The "changing rooms" sometimes are but in this case it's just a big room with stalls and lockers so privacy isn't really an issue.