The link itself doesn't. One link in your bio won't shadowban you, and there's no reach penalty for having one. That myth usually hides the real issue: if your reach dropped around the time you added a link, the cause is almost always something else you changed, like aggressive hashtags, follow and unfollow bursts, or a banned tag.
What a link can do is quietly stop working. If it's on a flagged shared domain, Instagram may strip it or mark it unsafe, so fans tap it and land nowhere. That's not a reach problem, it's a deliverability one, and it's fixed by moving to an address the platforms already trust.