How to release the work anonymously under the free licence? The goal is to release the work anonymously, but not into the public domain or as CC0, but rather under the Share-Alike licence. However, most of the licences require reusers to identify the original creator of the work, which they will be unable to do.
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.