Not sure if this plugin exists but I think this will be an amazing addition.
A lot of us have multi-sites where the sub-sites belong to customers. For privacy's sake you should not be able to login to their sites. I believe there are plugins for that but the thing is on rare occasion they may want you to login for support purposes.
So what is needed is a plugin which:
- blocks super admin access to sub sites
- Lets sub-site owner grant access to super admin for limited time just like it is done with wpmudev support