Currently it is visible to all, and editable by @staff. It can’t be restricted to admins only, but that would be a very helpful option.
Currently (whilst experimental at least) you can select who can see (and edit) custom links but it isn’t granular regarding editing global links.
Only /my links aren’t ok from those three. However, there seems to be a deliberate whitelist. I note that paths added by plugins are also not included (i.e. with the Custom Wizard Plugin /w)
You can muck about with this on Meta - it behaves exactly the same as for personal links.