I don't see why the fact that an answer is accepted would change anything. I've never heard of such a rule either. If anything, seeing a link-only answer accepted makes me more prone to converting it to a comment¹ (or deleting it if it's redundant). An accepted answer makes the question less likely to draw attention, and is more of a loss the day the link break.

As usual, the best course of action is you see a link-only answer is to provide real content, either by editing the answer or by writing your own answer. If the answer is recent, leave a comment asking the poster to provide more information; link to how to answer. If this is not possible (because the link is already dead and you don't know what was behind it, or because there's no one available with enough subject knowledge to provide content), or if another answer makes the link-only answer redundant, go ahead and convert/delete/flag.

¹ _{Since this is a moderator-only action, flag as not an answer unless you're a moderator.}

I don't see why the fact that an answer is accepted would change anything. I've never heard of such a rule either. If anything, seeing a link-only answer accepted makes me more prone to converting it to a comment¹ (or deleting it if it's redundant). An accepted answer makes the question less likely to draw attention, and is more of a loss the day the link break.

As usual, the best course of action is you see a link-only answer is to provide real content, either by editing the answer or by writing your own answer. If the answer is recent, leave a comment asking the poster to provide more information; link to how to answer. If this is not possible (because the link is already dead and you don't know what was behind it, or because there's no one available with enough subject knowledge to provide content), or if another answer makes the link-only answer redundant, go ahead and convert/delete/flag.

¹ _{Since this is a moderator-only action, flag as not an answer unless you're a moderator.}