The negative syntax for a backend config field <depends> condition

The core never uses this possibility, but there is a (single) example in a unit test:

Schema:

How it works:

Step 1

Step 2

Step 3

Step 4

See also:

An example of usage:

https://github.com/checkout/checkout-magento2-plugin/blob/1.1.10/etc/adminhtml/system.xml?ts=4#L401-L423

Another example: