How can two DELETE queries deadlock in Postgres?

You should instead ignore expired cache entries, so you will not depend on a frequent delete operation for cache expiration:

SELECT valueFROM cacheWHERE  key = 'Ethan is testing6460437'  and (ttl is null or ttl<now());

And have another job that periodically chooses keys to delete skipping already locked keys, which has to either force a well defined order of deleted row, or, better, skip already locked for update rows:

with delete_keys as (  select key from cache  where    ttl is not null    and now()>ttl  for update skip locked)delete from cachewhere key in (select key from delete_keys);

If you can't schedule this periodically you should run this cleanup like randomly once every 1000 runs of your select query, like this:

create or replace function delete_expired_cache()returns voidlanguage sqlas $$  with delete_keys as (    select key from cache    where      ttl is not null      and now()>ttl    for update skip locked  )  delete from cache  where key in (select key from delete_keys);$$;SELECT valueFROM cacheWHERE  key = 'Ethan is testing6460437'  and (ttl is null or ttl<now());select delete_expired_cache() where random()<0.001;

You should avoid writes, as they are expensive. Don't delete cache so often.

Also you should use timestamp with time zone type (or timestamptz for short) instead of simple timestamp - especially if you don't know why - a timestamp is not the thing most think it is - blame SQL standard.