http://i3xi5qxvbrngh3g6o7czwjfxwjzigook7zxzjmgwg5b7xnjcn5hzciad.onion/proposals/348-udp-app-support.html
We expect to rely on common optimizations to the performance of Tor circuits, rather than looking to make specific changes that optimize for unreliable datagram transmission. This document will briefly discuss UDP for onion services below.
It's worth planning for this as a way to evaluate the future design space, but in practice we are not aiming for UDP onion services yet.