Maimoto Keisuke is still live atm but the gist from livetranslations is this
Basically Maimoto is having this bad health symptoms since like, 3 years ago while streaming in Shizuoka. This symptoms persisted up until this year to the point that the Doctor be like "old man, ya gotta stop."
He has a bit of issue of telling it to management because he still wanted to stream, but (I'm guessing here) Koshien this year is more than enough to make him realize that his health issue/age is truly fucking him over.
So it isnt a graduation, rather, starting now he will not participate in any events and on September, he will take a indefinite hiatus in order to get treatment and all that stuff.
I'll edit this post if more info/live TL come up.