In one project that i worked to we constantly got on planning what we can estimate and what not and if that's the second, we could mark something as dependent o

In one project that i worked to we constantly got on planning what we can estimate and what not and if that's the second, we could mark something as dependent on some other task, or create "analysis" version of this task and estimate how much time we need to spend to be able to break down it do manageable chunks and investigate all the unknowns and then we planned it as any other task. we had some vague plannings, but those where in tshirt sizes or we just assigned numbers relative to other unknown tasks, so in long run management could roughly estimate project in months and manage risks