I’ve witnessed my parents’ Time Machine backups (the simplest backup system of all time) sitting stale because they unplugged the hard drive. They are ultimately responsible for paying attention to the little icon that tells them that the system is overdue for a backup.

I agree that there might be a bug if the exclusion list was modified without user intervention, though I don’t think this blog post convinces me that that’s guaranteed to be what happened. For all we know the author just didn’t notice this exclusion before.

And, again, I think excluding OneDrive and Dropbox is sensible. Most users are not intending to duplicate those backups.