Both Dropbox and OneDrive default to "online first" for most users (including Dropbox on macOS which has moved itself into File Provider). It is a technically sound and sane default for Backblaze to ignore these mounts, especially given their policy not to backup network drives. They really should have informed legacy users about it.

Technically speaking, imagine you're iterating over a million files, and some of them are 1000x slower than the others, it's not Backblaze's fault that things have gone this way. Avoiding files that are well-known network mount points is likely necessary for them to be reliable at what they do for local files.

It's important to recognize that these new OS-level filesystem hooks are slow and inefficient - the use case is opening one file and not 10,000 - and this means that things you might want to do (like recursive grep) are now unworkably slow if they don't fit in some warmed-up cache on your device.

To fix it, Backblaze would need a "cloud to cloud" backup that is optimized for that access pattern, or a checkbox (or detection system) for people who manage to keep a full local mirror in a place where regular files are fast. This is rapidly becoming a less common situation. I do, however, think that they should have informed people about the change.