Infinity Saga: 10 Unresolved Plot Points From MCU's Phases 1-3
6. Why Did Lady Sif Disappear?
From a logical standpoint, the answer to this question is that actress Jaimie Alexander was busy working on NBC TV series Blindspot. However, the MCU itself has never addressed Sif's absence in Thor: Ragnarok or why she wasn't among the Asgardians battling Thanos in Avengers: Endgame.
The last time we saw the hero, she was leaving the Reality Stone with The Collector, and while we would be tempted to say he might have captured her, Volstagg certainly didn't have any issues finding his way home again. At this point, it feels like the only logical explanation is that she may have been killed, but if that's the case, it's a real waste of the character.
Marvel Studios frequently teased Sif having feelings for the God of Thunder, so perhaps Thor: Love and Thunder can address that and bring her back into the fold? Fans are certainly anxious to see her again, and as things stand, it's crazy that an Asgardian who was key in Thor's early adventures has now vanished without any sort of explanation!