You are more or less. LiMux started in 2004. Last two points on its long timeline[0]:

* November 2017 - The city council decided that LiMux will be replaced by a Windows-based infrastructure by the end of 2020. The costs for the migration are estimated to be around 90 million Euros.

* May 2020 - Newly elected politicians in Munich take a U-turn and implement a plan to go back to the original plan of migrating to LiMux.

[0] https://en.wikipedia.org/wiki/LiMux

Last time I've heard about it, it ended with users (i.e. bureaucrats) complaining about missing features and/or differences between MS and the open source alternative and forcing switching back to MS

It was Munich - and it only happened once.

I shared the excitement, but as so often it was only executed half-way. In essence, instead of recreating processes from the ground up to fit the new reality, they tried to make everything as beforehand. Unsurprisingly, that was a huge uphill battle - in the end they spend more money than beforehand and had a lot of trouble in maintenance etc.

(Regarding the 2020 public announcement; nothing has happened since then IIRC so I would not count that in - just talk no actions)