Regarding deceased creators and Highschool of the Dead, the manga's writer, Satou Daisuke, passed away back in 2017; the series' artist, Satou Shouji, is still alive (the two are not related). While the issue has only ever been lightly addressed, Daisuke was seemingly the driving force behind HotD, leading to the decission to suspend the series.
Amongst a few other comic works, Satou Shouji's Triage X remains ongoing, with 28 volumes in Japan and the 27th recently released in the US; the series also received a not particularly amazing 10-episode animated adaptation back in 2015. Satou Shouji also remains an active doujin artist, working under the name Inazuma and his one-man circle Digital Accel Works.