What is a bygone relationship?

Could you please elaborate on the term 'bygone relationship'? It seems to imply a past connection that is no longer present, but what exactly constitutes such a relationship and what might lead to its demise? Is it simply a matter of time or could there be specific factors that contribute to a relationship becoming bygone? Additionally, are there any lessons to be learned from bygone relationships, and how might they impact our future interactions with others?