I didn't believe you until I went and checked the SDC policy and you're right.

Shocking change. Devalues one of the biggest perks of F and when combined with the fare class restriction basically makes SDC in main cabin pretty much impossible to use unless you already bought super close-in tickets in high fare classes (yes, I know available for sale is different than available inventory, but on most routes anything below an H fare was already dead for SDC).