As per my quick research 1 2 sometime around 1800 to 1920 European cities stopped erecting and started removing city walls. My understanding is that a lot of the laws regarding Eruv was assuming that most cities had walls, and those walls could be used for the Eruv. Many places nowadays have an Eruv by incorporating buildings wals and using an almost invisible string, with permission from the city government, to close the gaps in certain places. However, smaller communities, like many post-Holocaust Europen cities do not have an Eruv.
What did ~10M Jews in pre-Holocaust Europe do? Are there sources of them using a shabbos girdle? Did many cities allow string Eruvs to be built? Were there a lot of Shu"ts relating to the changing of the times?