Backblaze silently redefines 'unlimited' backups and users discover it's not backing up Dropbox and OneDrive

I used to use Dropbox for cloud backup of important files, but I haven’t in years. However, I still have the Dropbox folder and I still have important files and receipts in there, more current than the last sync to Dropbox which was many many years ago.

This is quite concerning to say the least.

roughly six months ago, Backblaze enacted a silent change that made its backup app stop uploading local data synced to "OneDrive, Google Drive, Dropbox, Box, iDrive, and others."

How does it know? I assume the shittiest way, by just having a deny list of path prefixes.