Wikipedia:Articles for deletion/Letter of thanks
I did a quick Google search for "history of thank-you letters", and it does look like there is enough to pass  after eliminating all the how-to sites and focusing just on reliable sources about historical significance: https://www.google.com/search?ei=7V6ZW533K8LKjwTc9Kso&q=history+of+thank-you+letters&oq=history+of+thank-you+letters&gs_l=psy-ab.12..33i22i29i30l10.20410.29528..32351...0.0..0.143.1541.0j12......0....1..gws-wiz.......0i71j0i7i30j0i13j0i7i10i30j0i8i7i30j0i13i30j0i8i13i30j0i5i30j0i8i30j33i10.mdcWRKfMeJ0. I believe that treating the subject in terms of historical significance does make for an encyclopedic subject. (Thank-you letter strikes me as a better pagename than Letter of thanks
— User:Tryptofish, 18:54, 12 September 2018 (UTC)
Stance: keep
Cited policy: WP:GNG (Wikipedia:Notability)