I think it will always stay compatible on protocol level.

That tools does what I always think was possible to do, or am I missing something?

I think the problem is that they do not share the whole communication protocol/model with GUI/client and it has only feature to block something aposteriory masks, while the whole folder with all files is already shared with client’s device. The original syncthing database scheme also seems does not supporting this.

Technically, from a protocol view level, there is no problem just to ask a single file to download and share only it.

I think because of that they do not implement “selective sync” feature, i.e. when you need to sync only a slice of remote directory.