They previously moved from mysql to postgres. See for example https://www.yumpu.com/en/document/view/53683323/migrating-ub... (from the same author as the postgres -> mysql blogpost no less).

Uber moved from MySQL (used as traditional relational DB) to Postgres (used as traditional relational DB) around 2013.

They then refused to ugprade postgres from 9.2 to newer version (and most of the stuff they complain about is not a problem in newer version) and moved to MySQL, used as append-only k/v store (they call Schemaless) now.