I'm curious too. Unless the developer is really passionate about this I don't think a dev will risk (potential) compatibility issues or unexpected footguns to use this when the workarounds do seem to work quite well already. I just dont see the benefit but maybe it's because I am just not aware of a real world use case; happy to be corrected.

Elastic/Opensearch uses GET requests with a body for search, which is complicated or forbidden (not exactly sure) with the HTTP spec. Not all HTTP clients are willing to submit a body with a GET.

So opensearch also allows you to POST search requests, but those are uncacheable

QUERY would fit here perfectly - it's probably trivial for opensearch to add but it will take some time for clients to catch up.