That seems not true for me. I have that setting enabled, and I last updated my computer November last year, and before that in 2019. Just checked my imessage db file and it goes back to 2014.
What you experienced sounds worrying though, I wouldn't want to lose that db on a potential computer upgrade. Do you have any more details on it or what you think happened?
Actually, I might be thinking of the ~/Library/Messages/Archive folder, which used to contain a readable backup of your messages and which stops updating with the iCloud sync.
What you experienced sounds worrying though, I wouldn't want to lose that db on a potential computer upgrade. Do you have any more details on it or what you think happened?