Defaulting to false, the effect of this permission would be to display a warning on login with wording to the effect of:

You have the vanish.warnpermissions permission set. This is probably due to a * permission being set, please remove it and grant permissions individually.

Hopefully this might cut down on the people complaining about *.

That's a cute idea. But if people don't read the documentation when they start getting effects, silent joining, silent quitting, etc... why are they going to bother after this?

Then perhaps a link to a web-page with a more thorough description?

Because this actually tells them explicitly what they did wrong, so they've got a course of action rather than just symptoms.

They don't really have a course of action, though. Most users who are told to remove the * node react extremely negatively, as if we're pulling the floor out from under them. It would need to be a very thorough description of the solution, and would have to be plugin-specific, and would have to know the exact cause of the node being granted. Information that isn't necessarily going to be accurate.

eg. Some older Multiverse versions grant the * node to OP when running PEX/GM. Some buggy versions of PEX grant the * node to OP as well. Users could also have just granted vanish.*