But don’t you dare try to have an open discussion at a stand up meeting. The moment a useful discussion starts the scrum “master” will tell you to schedule a separate meeting to discuss it. Honestly, that just kills the flow of communication. I have never scheduled a separate meeting to continue the discussion. We just end standup, go back to our desks, and don’t talk to anyone.
Have you considered that if this didn't happen, it would be one of your co-workers on this thread, instead of you, and they'd be complaining that:
"Our scrum standups are supposed to be 15 minutes long, but they always take an hour or more, because people start going down some rabbit hole that doesn't concern half the team, and the scrum master won't politely interrupt and ask them to schedule a separate meeting. So much time has been wasted by this that it's unbelievable."
I have never scheduled a separate meeting to continue the discussion.
That can be hard -- many / some people would be worried it'd be impolite to leave and in a way say "the things you're talking about aren't interesting to me".
And they'll stay and feel frustrated, in silence.
The result has worked almost exactly as intended: there's breathing room to discuss issues or ideas that come up in the course of conversation, but discipline around keeping those conversations brief enough to stay on the rails.
IMO at previous companies getting told "this isn't the time for that discussion" really frustrated me. As the OP stated, these conversations never happened, and it's healthier to allow some time and space for people to talk off-script when they're all meeting together.