It happened around laicite so I can imagine that it was used against Christians as well. The laws were also not universally applied until much later around the 80's.

Maybe in the 1900's people jumped into the pool with the same pants they wore all day but that certainly isn't happening these days. Swimwear is specifically put on before going into the pool and people can take showers beforehand. Archaic laws like this have no real reason to exist other than enforcing Apartheid. You can find plenty of other archaic laws like it which aren't enforced at all

https://www.connexionfrance.com/magazine/weird-french-laws-that-are-still-technically-valid-in-france/133482

According to a 1910 law, again technically still in place in France, it is illegal to kiss on train platforms.

While it remains in place no formal penalty exists so you should be safe to share a goodbye kiss before catching the train.

The actual solution to the problem is obviously for people to take a shower before entering the pool and to wear clothes for swimming. Not to ban certain types of swimwear.