I had smart previews uploaded for Lr Mobile.
Since upgrading, the sync status in Lr shows those as counting against a 20GB minimum.
Did Adobe end the policy of unlimited storage for smart previews?
Smart previews are on your own computer/Device. The 20GB you are referring to is cloud storage space which LR Classic didn't have at all. With CC your images are stored in the cloud as opposed to locally with Classic. The photography plan has 20GB of Cloud storage, the full CC plan has 100GB and you can upgrade to a TB if needed.
No, smart previews shouldn't count against your allowance. Can I get you to report it at the Official Feature Request/Bug Report Forum at https://feedback.photoshop.com/photoshop_family/topics/new/add_details?topic%5B please, so the engineers can figure out if there's a bug here.