What will they use instead? Who the fuck knows! The article omits this crucial piece of information.
Whole bunch of shit going by different sources and the state itself from german, to supplement here
MS Office -> LibreOffice Exchange / Outlook -> Open-XChange / Thunderbird Sharepoint -> Nextcloud Windows -> Linux MS Active Directory -> Unknown, but currently Testing things Telephones use, among others, Kamailio, RTPEngine, Asterisk, GenieACS, Loki and Grafana For all the Software to do like specific work, i.e. the software that helps manage industrial permits or whatever, it's case by case with them trying to replace them with mostly web based solutions so they're OS-Agnostic.
They're doing this together with Dataport, which is a sort of special government structure in the sense that it does IT for the states of Schleswig-Holstein, Hamburg, Bremen and Saxony-Anhalt who share the costs. They've been at this whole thing of trying to make a FOSS standard software enviroment for years now, steadily improving, so things might actually be happening. Video conferencing should be Jitsi, that's already in the portfolio, the chat components will in all likelihood be based on the Matrix Protocol which is aswell, I think they offer an offshoot of Riot.
It's a good thing. That said, the way the german government works this only really includes the actual state level bureaucratic engines. Everything at the county and municipal level will also have to make the switch themselves so that's 83 more government entities that would have to do this before the state runs on FOSS.
And like with all of them in germany they're all flat out broke and can't get personnel for this so this type of project, if attempted at all, is usually headed by a 60 year old who's also the equivalent of a CIO because he once built an excel table with pivot functions and the general level of digital competency of the workforce is dire, as in people are printing out excel tables to do the calculations with a calculator and things of that nature.
I think the main issue that usually gets trod out is how Microsoft makes the most ergonomical and useable software which I think is an argument you can only arrive at if you've just literally used nothing else, ever. The supposed point is that large swathes of the work force in the public sector would be unable to cope with the new software and be unable to do their job, albeit I point at my printing out excel tables example there to say they already don't know how to use software so at least save on the licensing fees