The Tumblr messaging feature deprecated when integrated messaging arrived in 2015 was fan mail.
Tumblr introduced an integrated instant-messaging function on November 10, 2015. The feature allowed users to chat directly with other Tumblr users inside the service, bringing private conversation closer to the main dashboard and posting experience. As part of that change, Tumblr deprecated its older fan-mail system.
Fan mail had served as Tumblr’s earlier private-contact mechanism. Its name reflected the way users often sent messages to bloggers, artists, celebrities, or popular fandom accounts. Unlike ordinary public replies or reblogs, fan mail gave users a more direct channel for one-to-one communication, though its design was less like a modern live chat system.
Tumblr Live, Post+, and group chats belong to later or separate parts of Tumblr’s product history, so they are tempting but incorrect alternatives. The important distinction is that integrated messaging did not replace Tumblr’s blogging format; it replaced the older fan-mail messaging channel. The change also marked Tumblr’s shift toward a more conventional social-network communication model.