This is probably not the most simple feature request, and I do not know if it has been discussed in length, whether it could be implemented at all, etc. Still think it is worth to mention it as an nice idea.
I don't like to have all my notes on a mobile device which I haul through train-stations, trams and airports or other. The risk of loosing it (compared to loosing my workstation), and it not being 100% certain that nobody can unlock the device ever is simply to disquieting. Couldn't a switch be added to the mobile version of Joplin (e.g. Android) which enables/disables the following feature : do only download / sync any notes with the tag XXX, ignore all other notes, notebooks, etc. ... FOREVER ? In other words make sure that the bulk of my notes does never end up on that device ?
I am doing the same think right now with two separate profiles and import/export, but it's a pain.
Your turn, your comments ?
The feature you describe could be called selective sync, very similar to the existing selective sync in Dropbox where on each device you can choose which folders to sync.
On Joplin you would select which notebooks to sync. Not tags because their content could intersect.