The term "AO3 Mirror Exclusive" generally refers to fanworks posted to the Archive of Our Own (AO3) that are not available on other platforms, or works explicitly labeled by authors as being exclusive to the archive following platform migration. Conversely, it can also refer to works on other platforms that are mirrors of AO3 content.
It wanted me to see.
Authors use this to prevent their work from being indexed by search engines like Google or to avoid harassment from non-users. Content Policy | Archive of Our Own ao3 mirror exclusive