So I got an answer from Sakagami Souichi.

He said that he himself got copyright strikes by a website called muzie.

He is part of this website but they don’t own any of the copyrights so he is basically fighting against this nonsens himself right now.

If you encountered a copyright strike for your Unicorn Senpai Slap Let’s Play it will most likely be by that comapny. He raised an opposition and you can do so as well because again: This website does not own ANY copyright to the music at all!

I wouldn’t recommend to get your videos back up just yet but be aware that he is currently fighting against this problem.