Good find! I've tried searching for data in my custom fields in many ways but was unsuccessful. I'm not sure that's still a feature so I'm going to ask our product team and follow up here once I have an answer on ZCF.
Because this is a custom field in Zephyr, I don't think you can search for it using JQL. You can try "contains text" but I haven't had much success with it. If it's something you'll know you'll have to search for often, I would suggest adding a custom JIRA field or utilizing an existing JIRA field.
Just confirmed that you can can search custom fields in Zephyr Squad, but you would need to do this from the "Search Test Executions" page. Custom fields are searchable by name and the 'zcf' keyword is no longer needed, in the screenshot below, my custom fields are 'Order' and 'API status code':
The documentation is out of date and should be updated soon, thanks for calling this out!
Furthermore my custom field is enabled in the project but it isnt found in the column nor the search - Tried reindexing but no change - are the "Test Steps" tab custom field treated differently to the custom fields under "Executions" tab