The How to Answer help page has the following section, which mentions that links to resources can be unreachable or be taken offline permanently:
Provide context for links
Links to external resources are encouraged, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the target site is unreachable or goes permanently offline.
What this doesn't cover is the situation where a linked resource (page, article, video, blog, or other piece of content) is taken down, but the site itself is still available. This has occasionally been used as a pedantic justification to argue that link-only answers are fine, especially for cases where the linked resource is on a "too big to fail" large site: "Pfft like YouTube is going away any time soon!"
Consider modifying this final sentence to explicitly state that content can be taken down, irrespective of what happens to the site itself. Something like the following: (emphasis to highlight the change):
... Always quote the most relevant part of an important link, in case the linked resource is ever removed, or the target site is unreachable or goes permanently offline.
