Hopefully we get that for the side stories. He was such a true homie.
They left it out, in the novel Changhyun ended up sleeping over at Yuri's place after their night drinking and he slept in the same bed as Yuri, to which plush Mikhail chased him with a knife in the morning with Chucky playing on the TV. Changhyun was a huge third wheel in that last part but at least they got to say goodbye as they made the ritual to get back to their world with Mikhail. Regarding side stories, we will also not get anything from the other world, unless they decide to change it. I agree thought, there should have been a proper farewell in the webtoon as well.
I'm glad they met again but bruh imagine your best friend taking care of you while you're in the other world but got cut off immediately after. They should have included the farewell between the mc and his friend before leaving his world. That could have answered the best friend's question on who mikhail is.