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Here's Dropbox's writeup, see the table entry "Some types of files aren't supported on Dropbox for macOS."
https://help.dropbox.com/installs/macos-support-for-expected-changes
Attempting to open my Photos library results in the following error:
The library could not be opened. Main Dropbox 2.photoslibrary” is in Dropbox, and needs to be moved to a folder on your Mac to open.
I can't test it at the moment, but perhaps there's a clever way to symlink around the problem?