This is certainly a growing trend which was first started amongst end-users themselves, and slowly we've seen a few news media outlets also following suite. So far, only a very few government agencies have actually followed. Coming to mind are also The Netherlands. What is attractive for many organisations and agencies on Mastodon, is that...
The instance is almost 3.5 years old btw, which you can easily see from the instance admin account @itteam@social.bund.de. It just wasn’t used by many government departments at the time, mostly the data protection agency and the BSI. The @ltrlp@social.bund.de account itself is pretty old too. It dates back to before the whole Twitter debacle. I guess that’s also part of the reason why they decided to go full Mastodon, since they already have a lot of experiences with it.