Although the mayor who actually does it is socialist, it was initiated by a right-wing mayor (Jacques Chirac, who went on to become president) famously announcing he would himself swim in the Seine in the 90s (he didn't).
a) it was probably not cleaner 100 years ago, but that's the year when swimming in the Seine was banned because of pollution
b) Paris is not really downstream of any large city on the Seine and most of the pollution by far is produced by Paris and its suburbs, not by Troyes 150 km upstream.
What they are doing to reduce pollution is explained in detail in the linked article.
Although the mayor who actually does it is socialist, it was initiated by a right-wing mayor (Jacques Chirac, who went on to become president) famously announcing he would himself swim in the Seine in the 90s (he didn't).
a) it was probably not cleaner 100 years ago, but that's the year when swimming in the Seine was banned because of pollution
b) Paris is not really downstream of any large city on the Seine and most of the pollution by far is produced by Paris and its suburbs, not by Troyes 150 km upstream.
What they are doing to reduce pollution is explained in detail in the linked article.