Comment by pr_nik2

Legacy papers are very popular these days in econ, history, and political science. I don't have the time to dive deep into this one, but spatial identification of long-term causal effects is prone to errors. Specifically, the error terms in regression specifications capture unexplained variance that can be very persistent over long periods of time. These errors can also be spatially correlated across treated observations: https://voxeu.org/article/standard-errors-persistence