plasma-nm always sets a new connection's permissions property to the current user, meaning that only that user can use that connection: https://github.com/KDE/plasma-nm/blob/6cac88b61da1277dcb9a801e47f2d69829c68086/libs/handler.cpp#L234

Similarly the editor's default value for the `allUsers` checkbox is false (not really sure if that is ever used.. or is the editor window only ever initialized with values from an existing connection in a way that the defaults never come into play).

Apparently the approach recommended by NM creators is for the client to look at the `org.freedesktop.NetworkManager.settings.modify.system` polkit permission and if that is true for the current user session, default to a system-owned new connection (empty `permissions` property), otherwise default to user-owned.  This was recently stated in https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/743#note_952026 (and, as can be seen in that link I'm interested in making this logic work across all clients because when NM is switched to use IWD as the wifi backend -- a recent addition -- some features can only be enabled for system-owned connections.  But, I don't currently have a plasma setup to be able to try making the necessary plasma-nm changes)

For reference I documented what different NM clients do in https://gitlab.gnome.org/GNOME/network-manager-applet/-/merge_requests/98