(related: Should it be disallowed to flag deleted answers?)
I recently stumbled upon this question: Userscript to add a 'flag' button to deleted Stack Exchange posts
Someone is asking for a userscript, that allows him to flag deleted answers (a hobby of his). Meaning deleted posts that one himself can not see. The site itself does not offer such a functionality.
Two questions: Is this allowed behaviour? Should the site allow one to flag posts through API POST requests even though the website does not offer the functionality directly?