Chatbox visibility to me feels like it has a lot more in common with other Safety toggles, such as user voice and user icon. I feel like it makes sense to bundle it up in safety, as I imagine some of the reasons you'd want to disable it may also be reflected in trust ranking.
If it was on by default, and somebody used to VRChat wanted to turn the bubbles off, it's likely they'd visit safety first as that's where you disable other similar aspects.
Having it this way, the default safety setting should also probably be enabled for all ranks by default, as new players to the game who are mute may not realize nobody will see their messages by default.