I want to find very rare unique items like "Griffon's Eye", "Death Fathom", "Crown Of Ages", "Immortal King's Soul Cage", etc.
As far as I understand the drop algorithm in Diablo 2, it basically just depends on the Monster Level that allows the item to have the same item level to match the highest quality level of any unique item (Quality Level 87) and the treasure class.
Since Mephisto is level 87 he should be able to drop anything in the game (especially because he is an Act Boss). But he never drops items like Dimensional Shard → Deaths Fathom, Unearthed Wand → Deaths Web, Diadem → Griffons Eye, etc. Even though he is, as I said, level 87 and could theoretically drop a Sacred Armor [qlvl 85], Diadem [qlvl 85] or an Unearthed Wand [qlvl 86]. So that must mean that those mentioned base items are not in his Treasure Class, and he can't drop those base items.
Is it true Mephisto cannot drop those items?
If so, is there something else at play here besides TCs and Monster Level vs Quality Level?
(I took the mentioned quality levels from this Diablo Wikis page.)