Query autocomplete is a feature of the Kibana web interface, not of the ElasticSearch database itself. Which isn't to say that it isn't useful, but it's more of a niche utility than a core feature of the stack.
Maybe you're unaware OpenSearch covers Kibana's functionality via OpenSearch-Dashboards? Just like the rest of X-Pack under OpenDistro pre-name change
It's not exactly a niche utility for observability unless you plan on hand searching hundreds of fields. But of course see my other comment for a list of the other observability fumbles they've made.
Elastic chose a pretty great time to start to give observability attention, and OS didn't keep up there. Meanwhile search is becoming more and more focused on integrating semantic search (which Lucene isn't particularly excellent at)
What I'm getting at here is that there are use cases for ElasticSearch/OpenSearch beyond log collection and analysis; many of them don't involve Kibana at all.
