What happens if a DC goes down in a Postgresql, Patroni, etcd multi DC setup
Let me just copy the answer and following remarks from the main Patroni developer and maintainer:

You got nearly everything correctly. If the Site1 is down, Site2 will not promote due to the lack of quorum.  There is no way to automatically recover from such a situation.

The only solution would be to run one etcd node somewhere else, say Site3. You don't have to run Postgres there.  Keep the nu

Please note that the Postgres master doesn't necessarily have to run on the same node where the etcd leader does. They are independent from each other.

In any case, with only two data centers it is sometimes better to have two independent etcd clusters on Site1 and Site2, and run a Patroni standby cluster on Site2. If the first site is down, you can manually promote the standby cluster.

Automatic promotion in this case is not possible, because Site2 will never be able to figure out the state of Site1.