

Invitation-only Mastodon instances prevent spammers by controlling who can join and maintaining a trust-based network. Since every new user must be invited by an existing member, admins can track invitation chains. If someone turns out to be a bad actor, admins can ban them and also ban anyone they invited. This is like pruning the whole spam network. This discourages spammers because their accounts can be removed along with their entire invitation tree.
You are absolutely right on everything and remember it all perfectly.