I realize this is well over one year old, but I too would love the ability to sync by tag (with an option to include all linked files to that tagged file), especially when it comes to syncing with my Android phone. I don’t want to mess around with my folder structure by creating a “phone” folder and only sync that (which is a workaround). I would much prefer to add the tag ‘phone’ to those items I always want and need no matter where I am.
Adding my voice to the request. Syncing through explicitly excluding folders is too limiting. Adding allow-listing folders would be a step in the right direction. The ultimate experience would be allow/block-listing based on tags.