When screen types are defined in a Base project and thus imported/genereated into a working projectversion, it is still possible to change them. All changes, however, will be lost after generating the definition of the version.
When a mix of generated and new, specially for a version created, screen type exist one can easily make the mistake of changing a screen type in a working projectversion instead of in the base project where a screen type is generated from,
For generated domains editing it is already disabled in the SF, so why not of screen types. Or for all generated items for that matter.