Reasons for making this change:

RuboCop config files support an inherit_from directive.

When one inherits from a remote URL, a local version is download. For example: .rubocop-https--my-remote-config.com.

Links to documentation supporting these rule changes:

From RuboCop's docs

The optional inherit_from directive can contain a full url to a remote file. This makes it possible to have common project settings stored on a http server and shared between many projects.

The remote config file is cached locally and is only updated if:

The file does not exist.
The file has not been updated in the last 24 hours.
The remote copy has a newer modification time than the local copy.

Given this, is seems appropriate for git to ignore this file.