No - new field types isn’t related to having custom fields show on subtasks.
Just to be specific: it’s not that you can’t have custom fields on a subtask, it’s that custom fields are project-specific so you need to add the subtask to a project in order to access that project’s custom fields on the subtask.
You can add your vote to have custom fields available on subtasks (without having to add them to a project) here:
Also be sure to see this workaround which might work for you: