They're many, the oldest title I ever read was from the 50s, but unfortunately it's not available in this site, also you come to right person, since I love retro manga, here's some of my favs:
https://www.mangago.me/read-manga/heart_of_thomas/
https://www.mangago.me/read-manga/kaze_to_ki_no_uta/
https://www.mangago.me/read-manga/zankoku_na_kami_ga...... reply
.....Anything Tezuka Osamu. He is the mastermind behind Lost World(1948) and later with Astro Boy(1963).
https://www.mangago.me/read-manga/lost_world/
.....Now for YAOI, Kaze to Ki no Uta(1976) was first for serialisation, not sure why mgg has 2000 on it, but its wrong.
https://www.mangago.me/read-manga/kaze_to_ki_no_uta/
.....Then there is Zetsuai...... reply
there isn't a specific one. on 2011 when this site was created a lot of manga was uploaded to start it. so you'll always see the oldest dates back to 2011. reply