While there is a desire to bring the same features to both desktop and mobile versions of Joplin, it's difficult to provide a specific timeline for when certain features will be added.
As an open source project, the implementation of new features depends on the contributions of volunteers who work on a best effort basis. So, while there are no concrete plans or ways to have per notebook sorting on iOS at the moment, it may be something that is added in the future as more developers get involved in the project.
Hope this helps