The highest profile was in Munich where they did migrate >10,000 desktops to Linux and OpenOffice, but eventually they migrated back to Windows and Office:
https://en.wikipedia.org/wiki/LiMux
Lower Saxony:
https://www.zdnet.com/article/windows-scores-a-win-over-linu...
Vienna:
https://en.wikipedia.org/wiki/Wienux
Note that even when these projects succeed, the software users are running is invariably just Firefox, and often users have both a Windows and Linux PC side by side. There are no actual Linux apps being developed or maintained, in the sense of apps that use Linux APIs.
I can see niche cases, like laws where you want change tracking or very long reports but that does not seem to apply to most government employees. Somehow I feel I missing something big, maybe there is a lot of automation built around word documents?
A lot of computer users are drones. They have no conceptual idea or care about what they are doing. Moving stuff, even simple things, is crippling for productivity.
Now scale that up to hundreds or thousands of people. In local government people don't like to talk about the failures at all because there is fallout so we probably won't find the truth about what really happened but I suspect it wasn't all about politics and cash backhanders and MSFT investment.
If you really want to move off MSFT you will probably have to start again and run two completely isolated silos.