One thing that ressembles a little is in Dolphin-plugins we have KVersionControlPlugin and this was implemented dropbox:
It shows the current state, sync/synced, but it requires a full sync, not an on-demand download for individual files.
On Linux we simply don’t have an API/infrastructure allowing this. I have talked to Nextcloud people and they would need a solution for this too. This needs to be cross-desktop, file-system transparent.
The logical way would be to implement it using fuse, that would require some work, specific api-integration perhaps or network-protocol that could work over http/ftp/sshfs for instance. And the question would then how to expose the file state to the UI layer, my bet would be to use extended attributes then, that is if fuse allows it.