Though I no longer work for Amazon, I'm reasonably certain they use it from the description. Especially given I know for a fact that other more foundational services use it.

Why is it a "relic of years gone by"? Consul uses a similar, though more advanced technique[0]. Consul may not be as widely used as etcd, but I don't think most would consider it a relic.

[0] https://www.consul.io/docs/architecture/gossip

That patent is from when Kinesis Data Streams were originally announced to the public. Any reason not to think it uses it. Seems like it would have been a logical choice in the initial architecture and change is slow.