Is there an official policy on “rotten links” to external data? Shall they be removed, once found, or shall they remain for historical purposes?
A former homepage of Kammerorchester Carl Philipp Emanuel Bach (orchester dissolved in 2014) was turned into adverts for online casinos and porno web sites. I have removed the link (https://musicbrainz.org/edit/71637853), but now I wonder if it was a right decision.
You could use the date attributes and mark the relationship as ended. For historical purposes it could be good to keep track of the link, even though it doesn’t work anymore, and most of the time the link should still be accessible through the Wayback Machine.
If the link is hijacked for some nefarious purpose, I think it’s better to remove it. But it is better to turn it into an Archive.org link: https://web.archive.org/web/20071219182354/https://www.cpebach-berlin.de/
I’m not aware of any official policy, but it would certainly be good to agree on one.
Not sure it’s a good idea, I think it’s better to keep original link with an end date. If (yes this probably will never happen but who can predict the future?) Archive.org decide to change it’s link system we are doom :-/ + we keep a more clearer edit history IMHO.
I think the Internet Archive is probably the last organisation to not understand link rot. 
And the original URL is preserved in the archive link anyway.
Is it possible to add a date attribute to an external link? I am aware of dates only for internal links, that is relations between MusicBrainz entries. The external link editor does not contain any input fields for dates. If I am mistaken, could you please explain how to add a date to an external link?
It’s possible but it’s not straightforward. You have to go to the relationships page, then click on info next to the link you want to modify, then click on edit, and then click on the pen to edit the relationship. This is obviously much more complicated than it should be, but perhaps there’s an easier way I’m not aware of.
Thank you! We live and learn 
Vote added: Vote: Policy on rotten links
@regagain
it looks to me like this solution is only possible for the artist page.
Furthermore I have the question how to get the corresponding link from Archive.org.
It also works on other entities if you click on “info” next to the link.
Wayback Machine[page url]
There are several existing discussions about ended URL relationships, including a recent one:
https://community.metabrainz.org/search?q=ended%20URL%20relationship%20archive
Similar topics were tagged with #link_rot