How to release the work anonymously under the free licence?
The route that most people take, is to release it 'pseudonymously' rather than anonymously. So you just make up a pseudonym and use that in the copyright/licence statement.

A better approach may be to use the little-known CC-SA 1.0 licence, which was unfortunately retired by Creative Commons due to it not being used a lot. This licence requires a reuser to publish their altered versions with the same freedoms, but does not place the requirement to find out who was the original author.