

You haven’t missed anything; there isn’t any such place. There’s a bunch of suggestions on how you could patch such a thing together StackOverflow: Is there a way to list pip dependencies/requirements?. Basically either running pip
or querying the PyPI API to discover transitive dependencies. Sounds like a fun little programming project! 😀
I don’t know. I might say “Matrix” and run a private server? But that’s a bunch of IT work. It’s attractive to use a SaaS because you don’t have to do any long-term planning or hiring; just pay Slack a crazy amount of money and it all works.
Corps also like a commercial paid service because they get a contract with an SLA (even if it’s rare to actually get anything from these SLAs).