In our next release of Galaxy (hopefully later this week) we will deprecate Categories in favor of Tags. Role authors can add tags or keywords that best describe and categorize their role. Users of Galaxy will be able to search by tags.
Submitted PR 12456 to modify ansible-galaxy cli to reflect this change. It modifies the metadata template by removing categories and adding an empty tags array with instructions. And for the search command it replaces the --categories option with a --tags option.
The PR also adds --ignore-certs option to the install and search commands. This works nicely when --server is used to point to a host with a self signed cert.