Yes, Roblox is rated Teen. This is nothing surprising. Yet, losing out on communication with members where a large portion can be falsely banned without any method of appeals, is a bad idea.
I came to this opinion myself, although I do see your point. There are appealable bans, like bug-abusing/glitching, and accidental glitching which can be perceived as exploiting. However, most of these you need to communicate with a moderator somehow, which will now become impossible completely for users <13 with the removal of messaging etc. Also for some weird reason a lot of developers like to put the game rules in a Discord and not in-game, so you wouldn’t be able to see it anyways.
You do not need to be involved in development to make good suggestions, people want things in game and some of these can be good. Removing this entire category of users only benefits Roblox here unless they add some type of way for users to contact certain roles of a group like a form.
Adding onto this, these users should be able to SEE content on the group wall at the minimum, if not post there.