I could do that, but I that would also delete
system-source-registry system-source-registry-directory and default-system-source-registry
... which I feared might cause me access to bundled SBCL libraries.
i wanted to answer that you shouldn't be afraid, because it's implemented through an entry in asdf:*system-definition-search-functions*, but then i saw that it's not anymore, and i couldn't even find out in 5 minutes how ASDF finds SBCL contribs.
nevertheless, i have this in my ~/.config/common-lisp/source-registry.conf:
(:source-registry (:also-exclude "sbcl" "disabled-systems" "build") (:tree (:home "common-lisp/")) (:tree "/usr/local/share/common-lisp/source/") :ignore-inherited-configuration)
and the SBCL contribs are found.