Closed shadow roots are very rarely used and can hinder development/interaction with an element.
More information on closed shadow roots and why they may not be of much use can be found here.
This rule disallows use of closed shadow roots on all elements.
The following patterns are considered warnings:
this.attachShadow({ mode: 'closed' });
node.attachShadow({ mode: 'closed' });
The following patterns are not warnings:
this.attachShadow({ mode: 'open' });
node.attachShadow({ mode: 'open' });
If you wish to use closed shadow roots, you should not use this rule.