I was recently asked by a client whether they should have a leaving our website disclaimer that would pop up when a visitor clicked a link that left the site. My answer:
I understand why organizations do this, to try to prevent users from remembering content as coming from a content provider when it wasn’t directly from the provider. Legal sites and medical sites do this quite a bit. Some government web sites are regulated to do so. It is also a way to try to keep visitors on the site and not leaving for eventual parts unknown.
But that’s because it is a disruption to the content feed flow for the user, and users have varying opinions on that disruption. Speaking for myself, I’m not a big fan of the practice unless it’s really important to control the information the visitor gets that is “related” to your site, since you can’t control what is eventually on the link you are providing. So that’s really the question that needs to be answered internally – what’s more important, the perceived relationship of the linked content to your site(s), or less disruption for the user in the flow of getting content.
There are some industry niches where it isn’t uncommon, but for the web in general I don’t see it that often.