Is Arslan Senki (TV) Gaiden hentai?
No
Rated R (17+ (violence & profanity)) on MyAnimeList and not tagged as Hentai.

OVA · 2016 · 2 episodes · R - 17+ (violence & profanity)
Adventure, Fantasy, Historical, Shounen
It has been announced that Hiromu Arakawa's Arslan Senki manga will bundle OVAs in the fifth and sixth volumes, to be released on May 9 and November 9 respectively. The first OVA will include an original story supervised by Hiromu Arakawa, while the content of the second OVA has not been announced yet.
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