3. The Fate Of Max Mayfield
Thinking about it coldly and logically, Max’s story could’ve well concluded with season 4 as Vecna killed her to complete his ritual and open all four gates. In fact, she died on screen for a full minute according to Lucas. Keeping Max alive at the end of season 4 was definitely done on purpose, as seeing the character off for shock value could’ve added a new weight to the show’s stakes moving forward. So it's likely that Max has been kept alive for a reason.
One of the unanswered questions moving into the show’s final volume is foreshadowed by Eleven using her ability to search for Max's consciousness in the void and finding nothing.
One last point on the Talisman is that every human has an equivalent in the alternate ‘Territories’ universe and consciousnesses can shift between worlds by taking control of the body of their ‘twinner’. Unless, of course, one of the pair gets killed and becomes 'single natured'.
When Henry Creel confronts Eleven in the Hawkins Lab he tries to calm her by saying that those who he has killed aren’t gone, that they are “still with him” in his mind. If this is true, then Max must've joined them.
This is why Eleven can't find Max: because her body and soul have been separated. But if she keeps her friend's heart pumping, who's to say she can't reunite it with her spirit?