As long as we can create and delete tags, they will never be immutable, right?

The purpose of the forge is to be able to prevent this. Protected tags are usually a feature which provides a way to mark tags as untouchable, so removal would require a minimum level of trust to the repository on the platform. Otherwise, attempts to push tag deletions or changes for tags matching the protected pattern would be rejected/ignored.

Of course, the repository owner has unlimited privilege here, hence the last part of my prior comment.