We use labels to track where a project is in a workflow.
Is these a way to search for all tasks that have no labels?
Unfortunately it's not possible at this time. Feel free to vote on this suggestion. https://todoist.com/Vote/show/6108/
I'd vote for this feature as well.
I filter my tasks using labels and usually I don't label them right when I create them. the number of tasks usually pile up pretty quickly and I cant search through all my projects to find the tasks without label. I believe this is the most important feature to implement.