I heard it was being phased out. I don't remember which version. You probably can search it in release note.
It will be not available in the library in new version of MATLAB. But if it exists in a model from old version of MATLAB, it still works after the model is opened in the new version of MATLAB. You could copy and paste if you have to use it.
You could use MATLAB Function block instead.
Again, for now, it is just not available for drag and drop from the library. It is a way that Mathworks tries to guide you away from using it.