It seems dev-scheme/mit-scheme package has been removed from portage, it could be because of files containing executable stacks or something.
If you (like me) need the MIT/GNU Scheme, and prefer not to install packages unless there's an ebuild (thus there's emerge) somewhere, here's the Gentoo lisp overlay, but to my surprise it's not listed at overlays.gentoo.org. This overlay contains dev-scheme/mit-scheme-c, and no, it's not binary.
Simply clone it (and don't forget to pull periodically), and add the directory to your PORTDIR_OVERLAY.
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