This is not the only example of questionable logging with iMessage. I've long been concerned about iMessage/iCloud's tendency to store contact names and numbers you've had in your iCloud Contacts (or perhaps even not) long after they've been removed.
These contacts appear in the Recents list of the Messages app (iOS or macOS) whenever you start typing in the recipient field and there's no way to permanently clear this list. It keeps building, seemingly as a record of anyone you've ever contacted that either has or had iMessage or been in your iCloud Contacts (I can't tell which, possibly both) regardless of whether they still are. You can hide the contact by clicking a blue (i) button in the list and selecting "Remove From Recents" but contrary to claims this does not permanently remove it from the Recents list Apple is storing. It simply hides it from the current device until you "Erase All Content and Settings", after which the complete Recents list returns.
This is reproducible. You can (painstakingly) remove all the contacts you don't want from the Recents list, but if you reset the device or get a new device or Mac they come back. Therefore, Apple must be storing this list and you have no access to change, modify or permanently delete the list. It's a completely separate list from your iCloud Contacts. If Apple stores the information then it's available to law enforcement and the argument that this won't be accessed or used unless you're under suspicion of having committed a crime is a moot point. Privacy is privacy regardless of how much or how little you trust the motivations, intentions and capabilities of the state and law-enforcement agencies (in my case very little). If it serves no function or benefit, or even if it does, I should have the option of deleting the record permanently. As it stands I do not.
At the very least this behaviour is annoying. If I ever reset my phone or computer or get a new iCloud-enabled device or Mac I find a big Recents list containing contacts from years past I have no intention of ever contacting again and this makes it harder and slower to find the contact I do want from that list. At worst this is a scary form of tracking. A Recents list should contain only contacts you've contacted, you know, recently, and I should have the option of clearing that list from Apple's servers at any time. Contacts I haven't contacted or had in my Contacts list for years shouldn't be in that list!
I would like answers from Apple as to why this is occurring and I plan to contact them to report it and seek those answers. Personally, I think there's a disconnect between Apple's marketing and the reality of privacy concerns regarding its online services. Governments would (and do) go to extreme lengths to get the sort of information iCloud is using. Apple is aware of how much concerns around privacy could be a deterrent to sales so it's in their interest to downplay these concerns.