From your own link (the guidelines following the main body):

Label keys should begin and end with a lower-case letter and should only contain lower-case alphanumeric characters, the period character (.), and the hyphen character (-). Consecutive periods or hyphens aren't allowed.

i.e. while underscores (_) and slashes (/) are technically allowed, you shouldn't use them

I agree, you should not use them. But it does not say, you may not use them. First sentence in the Docker Docs: https://docs.docker.com/engine/manage-resources/labels/#key-format-recommendations

A label key is the left-hand side of the key-value pair. Keys are alphanumeric strings which may contain periods (.), underscores (), slashes (/), and hyphens (-)._

So, either merge or decline.

First of all: this is a linter, the intention is helping to build best practice Docker images, not just valid ones.
You yourself agree underscores and slashes shouldn't be used in label keys.
Secondly I'm not a maintainer of this program, so I'm not in a position to accept or close this request. What I can do is give some context why the rule is the way it is and why this change doesn't make sense.