0.25.0 and earlier used a custom search engine, but there was no point in maintaining it, as it didn't scale reliably and wasn't future proof. So there's no point in installing Elasticsearch for that version, however 0.26.0 requires Elasticsearch, and all future versions will do so. I recommend using a docker-compose with the official Elasticsearch image (version 7.x), but it can be hosted in any way on any machine that can be communicated with.
I also recommend not sleeping on 0.26, as it has better startup times, faster and better searching, more features, bugfixes and will be the way forward.