Answer:
The MAX6626PMUT#TG16 and MAX6626RMUT#TG16 are the RoHS exempted parts that contained tin-lead (Sn-Pb) solder balls internally in the flip-chip package that is otherwise a lead(Pb)-free package whose lead-frame is 100% matte tin over copper lead frame. Now, these RoHS exempted flip-chip SOT23 parts are discontinued because the leaded bump subcontractor supporting these parts cannot build with the product-specific materials.
There are qualifications ongoing using a lead(Pb)-free solder ball method of flip-chip SOT23 assembly, but the qualification is not certain. It is unknown at this time if Maxim will offer a SOT23 lead(Pb)-free version of the MAX6626.