I have a very mild jolt of anxiety every time I want to enter a new line in Teams or Slack, wondering if it will send a half completed message I will need to edit after the recipient has seen the half completed message, or it will enter a new line like I want it to.

The behavior also changes if you start editing a numbered or unordered list. Maybe that enters the "formatting tools" mode you mention?

If you're worried about this, consider a copy and paste from notepad.