It is now clear to me that #Unity messaging is deliberately unclear and confusing because developers have the legal right to avoid the retroactive payments if they do not update.
The Unity TOS used to provide that an updated TOS would not apply if you did not update your Unity software, and they only recently removed that language.
They now appear to be deliberately covering up that change.
https://www.reddit.com/r/gamedev/comments/16hnibp/unity_silently_removed_their_github_repo_to_track/