You used to be able to make an XML file with the search query and base64 encoded icon and put it in the search engines folder in your profile. It wasn't the easiest way for non technical users, but once you knew how it wasn't bad. It made it easy to backup, together with bookmarks.html.
As of at least Firefox 69.0.3, you can right click most query fields and choose "Add a keyword for this search". It works about the same in e.g. Opera and Vivaldi.
It's unclear if the "intent to deprecate OpenSearch" also means removing this functionality.
