In my experience with Slack, vendors are always locked down. A good practice is to create a totally separate Slack instance where vendors and contractors can talk freely with employees. If non-emoloyees are on company Slack, they can pick up IP, be witnesses to harassment, be exposed to PII leaks, etc.

Locking down channels is tough. As a company grows, there can be immense confusion when channels pop up all over (especially duplicates). Having some friction to creation can be helpful, but that could be simple public “shaming” instead of getting IT / HR in the mix.

When HR thinks its job is to administrate, the company has lost its chance at having a real culture. It sounds like using MS Teams is like having HR administrate over everything you say at work.