One of the more common mistakes (and quite understandable, since it takes an extra click to see that there's an 'ended' option available) is new and not-so-new editors removing relationships rather than marking them as ended.

Like we do with track capitalisation (MBS-13711), we could display a warning message when editors remove a link.

Perhaps with this text:

{{}}Did you know: Relationships that are no longer valid should be set as 'ended' instead of being removed, which is done via the pencil icon. Relationships that were never correct can be removed.