I came across the same post:
yarn es uses snapshots from our nightly unified snapshot jobs. These are placed into an S3 bucket with a TTL, so they don't last forever. Since it's been quite sometime that 6.4 has been released, it's not expected to be available.
There are a couple options.
You can run Elasticsearch from source on the same branch. If you place the
elasticsearch as a sibling to
kibana you can run
yarn es source and it will do all the building/running for you.
Alternatively, and probably the simplest option, you can download an Elasticsearch build and run it on your own.
Thanks for your help! I think it would be better to put it into the CONTRIBUTING doc.