Sync Desktop now that Dropbox no longer supports symlinks
So far as I know, there are only two possible ways to do this, and neither ways are available as of the latest macOS (Catalina, 10.15) and the latest dropbox updates (EDIT: it turns out that there is a workaround for the problem with the second way!).


The first way is to create a symlink within Dropbox that points to a folder outside Dropbox and will trigger Dropbox to sync its contents. This is how I used to have things until the latest dropbox updated (apparently triggered by my updated to Catalina) broke this method. As the questioner notes, this method is no longer supported by Dropbox.
The natural workaround (the second way) is to (i) place the desktop folder contents in a folder within the Dropbox folder, then (ii) effectively turn the actual desktop folder (stored in the user folder) into a symlink (e.g. per these instructions; EDIT: this is also effectively what Alex Szatmary suggests). Unfortunately, this by itself results in only partial success: on Catalina, so far as I've seen, after restarting the computer, the desktop reverts back to normal desktop configuration (no longer functioning as a symlink pointing into the corresponding desktop folder in the dropbox folder). EDIT: But there is a workaround: once the desktop folder is turned into a symlink that points to the desktop folder in the dropbox folder, go to the "Home" folder, and 'get info' on the Desktop alias/symlink and check the box "Locked" in order to lock the folder. This way it will not be deleted and replaced with the normal desktop folder upon restart and logging in.