This can now be done. You can crate tasks without a date using the normal create task modal or the new quick create option. To use the new quick create feature open a project and go to the task list page. Then type into the "Quick create task" input at the top of the page. Tasks will be added without a date in the list page under a heading called "No Date". You can then edit the date and other details by clicking each piece of info or by clicking on the task title to edit all task details.

We're working on a big update to how we handle tasks and this will be part of it.