Is the municipality BfS Nr. unique over time?"
In SMMT: The Swiss Municipal Data Merger Tool Maps Municipalities Over Time

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

library(SMMT)

No.

Example

The bfs_nr 3955 belongs to the municipalitiy Landquart (GR). Until October 1977, this bfs_nr 3955 was also in use but for different geographical entity, namely the "Gemeindefreies Gebiet Maienfeld-Fläsch".

Consequence

To join two datasets of two different time periods, one might think that the bfs_nr can be used to do the join. However, this is not possible because the bfs_nr can be reattributed over time. Thus, a simple bfs_nr based join is not possible.