That passive was removed before she hit live, and it's not 'hidden' either.
They had it in place to discourage her from building as some unkillable super-tank with still decent heals (Which removes the counterplay of 'Shoot the Medic"), but with the short ranged heal keeping her close to the action, along with the percent health cost, they decided that she didn't really need it (At least not for now).
Only thing about building defenses on Soraka is that you have to keep in mind that extra health doesn't help her much, and that you should focus more on resistances (Like a frozen heart in place of a Randuin's).
EDIT: And honestly, getting less defences out of a defensive item is pretty significant, while most 'hidden passives' are mostly inconsequential to gameplay: there is no way they'd make something that had a serious affect on her build path NOT be mentioned in the tooltip somewhere.