A time travel conversation on trying to figure out the rules of it in the MCU (going off Marvel Studios' properties, thus excluding Marvel TV rules for the sake of the conversation). If anyone wants to join and/or enlighten me, by all means! :)
Time travel can be very confusing. When Avengers: Endgame came out, there was many a discussion about how it really worked, and who did what, etc. However, many of our users seem to believe that A) the timelines the Avengers visited in Endgame always existed as parallel universes, or B) The Avengers went back in time, and in doing so created an alternate timeline/You cannot go into the past without immediately causing a branch. If I'm not mistaken "B" is what is used widely here on this Wiki, thus treating Hulk's conversation as being with an alternate Ancient One, as opposed to the main one, same with Thor talking to his mother; the wiki considers it to be an alternate Frigga, of an alternate timeline ("2013/Time Heist") created by Rocket & Thor.
However, the first Loki episode ("Glorious Purpose") informs us that there is one timeline (The Sacred Timeline) and that any branches formed are deleted before it reaches the Red Line, after which it cannot be erased. That first episode also has the TVA state that the branch in 2012 occurred after Loki escaped with the Tesseract, thus making it sound as if everything before that (Ant-Man giving Tony cardiac issues, Cap fighting Cap, Cap saying "Hail HYDRA" in the elevator) was all part of the main Sacred timeline. ... How is that possible?? HYDRA agents like Rumlow & Sitwell clearly never had Cap (Steve Rogers, obviously) tell them that in 2012 in the main timeline, as it would contradict the events of Captain America: The Winter Soldier two years later.
The second Loki episode ("The Variant") reveals even more. Going back in time does not automatically create an alternate/branched timeline. So you can indeed go into the past and under a specific set of criteria, not branch out. Loki & Mobius go into the past of the main timeline, to 79 A.D. and witness the eruption of Mt. Vesuvius in Pompeii. And as Mobius discovers, this does not branch the timeline. Loki even says he could go to Asgard right before Surtur initiated Ragnarok & knock Hulk off the Rainbow Bridge (which obviously didn't happen in Thor: Ragnarok) and that, due to this little 'loophole', it would not branch out the timeline whatsoever but would be part of the Sacred one.
This, coupled with the Loki episodes revealing that the 2012 branch started when Loki escaped with the Tesseract and not when the 2023 Avengers arrived there, could also mean that Rocket & Thor, having arrived in Asgard right before the Dark Elves' attack, were free to do whatever and it would not branch out the timeline... meaning that there is no 2013/Time Heist timeline, and rather Thor & Rocket had gone to Thor's actual past. I feel like someone's gonna say "Well, maybe the Dark Elf attack isn't an apocalyptic event" but the TVA report file list the the D.B. Cooper hijacking somehow counts as apocalyptic (somehow)... so if that counts, so would the Dark Elves' attack on Asgard, I'd say.
Speaking of going to the past, there's Steve Rogers. It is a travesty that both the directors & writers of Endgame cannot agree on how his story ended. The writers are adamant Steve went to the past of the main timeline and lived a life there, in the shadows, and that he was always Peggy's husband. The Russos state that Steve went back to the 40s and immediately branched out, free to live a life & change history as he saw fit. I don't see Marvel Studios having a "third party" (be it Kevin Feige or one of their shows) clarifying this, hence why The Falcon and The Winter Soldier never addressed it either. Heck, they didn't even tells us what happened to Steve after he came back & gave Sam the shield.
However, with what we've seen so far from Loki, it seems implausible Steve's branched timeline could go on for 70 years without nearing Red Line. The 1985 Wisconsin branch in The Variant began to near Red Line in about an hour or so (judging by the weather outside and the Renaissance Fair still going strong) so it's not like the TVA could've allowed Steve to live in a branched timeline, wait until he returns to the main one as an old man, and then prune the branched one he lived in. It also seems probable that Steve could have inadvertently found a similar loophole as "The Loki Variant" and found a way to go into the past of Sacred Timeline without branching off or alerting the TVA, thus they remain unaware of his actions.
So, to "sum" it up: At the moment, the wiki is operating under the rules that 2012/Time Heist formed when the Avengers went back in time, but the Loki show reveals that the alternate timeline didn't exist until Loki escaped with the Tesseract. How then do we sort out the Avengers Time Heist stuff that occurred before Loki's escape, as it would suggest it was part of the main timeline? Also, If you can go to the past right before an 'apocalyptic' event, then did Thor & Rocket go to the actual 2013 past & speak to actual Frigga, instead of creating 2013/Time Heist?