You could do unintentional linking if you don’t want to be subject to DMCA claims, like putting in the description
“We do not provide support for external projects that use our code, like shadow-website. Please contact them for further assistance.”
or
“If you’re using external services like shadow-website, please say so while reporting issues/bugs.”
This gives off an air of legitimacy to the shadow-website to anyone who’s interested in looking at mothership, but doesn’t explicitly associate it with mothership.