Is Girls & Panzer: Fushou Akiyama Yukari no Sensha Kouza - Extra hentai?
No
Rated PG-13 (Teens 13 or older) on MyAnimeList and not tagged as Hentai.

Special · 2018 · 1 episodes · PG-13 - Teens 13 or older
CGDCT, Educational, Military, School
In honor of Girls & Panzer TV & OVA receiving a 5.1ch Blu-ray Disc BOX set, a new Akiyama Yukari's Tank Course "Extra" was made with nearly a half hour worth of content. A short ~7 minute watermarked 'preview' version was available on Bandai Namco's official YouTube Channel on September 29, 2018.
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