Is Kyoukaisenjou no Horizon Special hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

PV · 2018 · 1 episodes · PG-13 - Teens 13 or older
Action, Fantasy, Sci-Fi
The Blu-ray Box set of Kyoukaisenjou no Horizon is slated to have an "all-new bonus animation." (Source: ANN)
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