[blueelephanttea]

> Anyone know if it's still a drop in replacement for Elasticsearch?

As you point out it was forked a number of years ago so it started from the same place (7.10). Elasticsearch is now on 9.0+ and has 27,000 more commits than OpenSearch. So I doubt it is a drop-in replacement anymore.

I have no idea how many of those 27K commits are key features, but it is clear divergence.