Yeah, or local governments, or governments in general, seems strange to me too. But then probably they'd use Facebook/Instagram/Whatever instead of Twitter if Twitter wasn't available, and seemingly it's impossible for them to use something that isn't built like a panopticon that also wants to slurp up as much data from their users as possible.
There should be a law mandating government agencies to not limit their electronic communications to private third party platforms.
If they post something on Twitter, or worse, Facebook, they must also provide a copy of the post on their own website. Some kind of online government gazette, if you will.
https://en.wikipedia.org/wiki/Government_gazette
How would you define it so the internet wouldn't also be considered a third-party platform?
ISPs and infrastructure in the Internet are generally considered common carriers unlike social media platform who can do whatever they want. I don't understand why you think this would be hard to define at all.