GraphQL mutations have been brought back. Unfortunately we had to remove them a while ago, because the implementation had a critical bug and it was not possible to solve this bug with the GraphQL library we use.
A few days ago a new version of this library has been released and it was possible to fix the bug. Squidex has mutations to create, update, change and delete content items. These are the most important functions when dealing with content. It is not planned to release management mutations with GraphQL as we already have the REST interface for that.