It would be very useful if there's an event for logging permission / node changes. I know that there's already the NodeMutateEvent but it doesn't provide the info which player changed the node.

As an alternative it would be nice if you add the source of the node change (player name or console) to the NodeMutateEvent.

This functionality is already provided by the ActionLogger - this is a mirror of the data used by the /lp log command.

Have a look at: luckperms.getActionLogger().getLog()