The Matrix homeserver Conduit supports Server-side key backups

Matrix is a secure, private, decentralized, open source messaging platform (https://matrix.org)

Conduit (https://conduit.rs) implements the Matrix specification in Rust, which makes it very lightweight and performant.

The server-side backups are not merged into the master branch yet and Conduit is still a bit away before one can use it like a regular Matrix homeserver (Federation is still missing).