philosophically Historical Lock questions are deleted
Visual difference between historically locked and legitimate questions looks quite subtle to me, even though I have few years experience on the site. It can be even harder for inexperienced readers to tell the difference.
Given that historical lock is applied mostly to (inappropriate) questions having large amount of views and ranked highly in web search engines, this creates wide open broken windows. Inexperienced site visitors coming from web searches at h/l questions think that these are legitimate and try to ask similar ones.
In order to help newcomers see the difference, suggest to use pink background when rendering historically locked posts - same that is currently used to render deleted posts displayed to 10K users.
Related feature request at MSE (hat tip to durron597):