Instagram blocks searches for certain tags that are regularly used to share inappropriate content. Sometimes these tags seem completely unrelated, for example the tag “fashion” gets blocked from time to time, but it is always temporary.
The best way to confirm if a tag has been blocked is to view its instagram page, for example:
https://www.instagram.com/explore/tags/sexy/
Scroll all the way to the bottom and you should see a message stating the posts have been blocked.