Now, the https://github.com/linuxmint/mint-y-icons/blob/master/README.md still says "The folder icons originate from the Arc icon theme". I don't know what "Papirus inspired" means exactly here, but i feel this credit section probably needs some update(s).
Check this https://github.com/linuxmint/mint-y-icons/commit/7f804620de2955d7d91e60ddfa1bdfb1fcd967b0 commit.
Now, the https://github.com/linuxmint/mint-y-icons/blob/master/README.md still says "The folder icons originate from the Arc icon theme". I don't know what "Papirus inspired" means exactly here, but i feel this credit section probably needs some update(s).