Isn't that just an old fork of Elasticsearch?

It is a fork, but not old; they have ongoing commits: https://github.com/opensearch-project/OpenSearch/commits/mai...

Plus, given that AWS is currently hosting Open Search, they are not incentivized to sit on their laurels when it comes to modern features or stability

Went from ES and sharding hell to less of a sharding hell with OS on AWS. I've been looking for a replacement ever since first friday evening sharding party with infrastructure team.

"sharding party"

Man, that made me laugh. I'm using that.

yeah, you haven't lived until you curled in blind rage _cluster/allocation/explain and _cat/shards?h=index,shard,prirep,state,unassigned.reason | grep UNASSIGNED every few seconds; In production party, with tarantulas on your back asking for status, of course.