Reading what's available, it seems like this is more for custom data, but I can't find anything about it preserving or reading existing data.
Specifically, I'm looking for a solution to read & write file tags across macOS & iOS.
That's a matter of the Dropbox client syncing behavior, and not the API, so I'm afraid I can't offer insight on that.
Please check out the other parts of the forum e.g., Find answers > Syncing and uploads, or open a support ticket, for questions for the Dropbox product itself.