I've been reading the documentation for 2.4 and can see that ElasticSearch is a required module to upgrade - we'd been using a third party search module which we will continue to use for 2.4 too however you can't physically upgrade to 2.4 unless you have an active connection to ElasticSearch.
My question is: is there a way to upgrade to 2.4 without installing ElasticSearch on the server? I find it hard to believe that Adobe have decided to scrap the MYSQL search and replace it with one you have to pay and subscribe to.
Any advice is greatly appreciated!