Green’s Dictionary of Slang

a letter concluding a relationship.

1984		J. Dailey Silver Wings (1985) 279: He wrote his wife a Dear Jane letter.
2003		‘Advice Diva Tara Solomon’ column 7 May in Miami Herald 🌐 Several years ago, I returned home to find a ‘Dear Jane’ letter on my bed. I was absolutely shocked, as the night before, ‘John’ told me he wanted to marry me.
2003		Troynika ‘Dear Jane’ (poem) on Starlite Cafe 🌐 I will love you forever/ even though I received a Dear Jane letter.