For years, I've kept my 286GB Photos library backed up via Dropbox. But as of MacOS 12.5, this has been disallowed, per Apple's policy.
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?