This is something we're going to keep an eye on but at this stage I don't think it's fair to start making changes to support OpenSearch given its infancy and very early development status.
This particular error is only the tip of the iceberg. Due to changes in the Elasticsearch API over many years, this is only one of many version checks that we do that would actually send OpenSearch down incorrect code paths as it stands.
These issues aren't insurmountable but of course depending on the direction they decide to take it and how much they decide to diverge, it could be the first of many issues.
At this moment in time we only support officially recognised distributions of Elasticsearch but we will keep an eye on OpenSearch as on the surface it is a good initiative being done for the right reasons.
Actually, XFES works with ElasticSearch 2.x and above.
Does this indicate that it actually supports OpenSearch 1.0.0-SNAPSHOT like supporting ElasticSearch 6.x despite the version number incompatibility?
At this moment in time, yes, it is just the version number issue as far as we know.
I will endeavour to at least try OpenSearch by hacking the version number checks but on the whole it may be too early to support. But at least if we run into anything upstream then we can feed it back.
I would double check that XFES is not doing something based on that version number - in other plugins we see checks that say "if version is <= 6 do X, otherwise if version <= 7 do Y", so 1.0 becomes 6 and that's not good cause it's compatible with 7.