From what I recall, a tweet says he died in his sleep.
Edit:
January 2022
Harsher in hindsight?
Then again, the IRL Nice Nature was already really old by the time this comic was made, his passing away was inevitable.
LatelyNature has been spending more time lazing in the sunSince it's becoming inconvenient to wash and dry her long hairI cut it short awhile agoMy belovedRecentlyWanna eat?Trainer-san...Has been calling me a name that brings back memoriesAh...