There's also at least one major city that has a public chat bot, New York (a few years ago they seemed to have gotten in trouble for telling businesses they were allowed to take tips from employees). But yes, it's public, so obviously suffers from the same fault that they all do.
Anyways, all the services mentioned in this thread, and many more, have been put together in a basic python library that lets you interface with any of them anywhere. Probably, to be safe, I recommend only using this behind a VPN:
https://github.com/TomCasavant/openllms
And also the Maubot plugin for matrix:
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