During the events of Loki (2021) 1x6, the sacred timeline splits. This happens at the end of time (or something like that), meaning it doesn't take place at a specific time. So in the MCU timeline, when does the timeline split? was the sacred timeline just always split, because the splitting happened at the end of time? Or does it split in 2012, when Loki escapes the Avengers and creates a branched timeline? Or maybe in 2023 when the Avengers go on the Time Heist and cause Loki to escape? Or am I just missing something?
They have always existed. 'He Who Remains' merged the timelines into one after fighting with his variants, but he created the TVA to stop any splits from happening. Think pruning a branch. So all time lines were basically the same. At the end of Loki when they start splitting is when they become very different universes.
There is a scene in WandaVision at the very end when she's reading the Dark Hold book, she hears her son calling. The theory is she is hearing her son from another universe because the timestamp of it happening and the timelines splitting in Loki are exactly the same.
Before the events of WandaVision, which took place in the fall of 2023
Marvel Studios’ The Marvel Cinematic Universe: An Official Timeline gives us the official order of events in the MCU. The book shows that He Who Remains met his end at Sylvie’s hands in the Loki Season 1 finale before the events of WandaVision (Fall 2023).
The book tells us that the events of Loki Season 1 happened before WandaVision, Spider-Man: Far From Home, She-Hulk: Attorney at Law, Spider-Man: No Way Home, Doctor Strange in the Multiverse of Madness, and Ms. Marvel. This makes a lot of sense and explains why those shows have so much multiversal shenanigans going on — because He Who Remains was offed by Sylvie.
According to the official timeline, He Who Remains' death allowed Wanda to peer into other universes.
Fall 2023 | Darkhold Corruption
In a remote cabin, Wanda Maximoff studies the pages of the Darkhold that she took from Agatha Harkness. Its pages contain forbidden methods of traversing universes, and with He Who Remains now gone, and the TVA no longer maintaining the Sacred Timeline, the doorways are open. Wanda uses the Darkhold to pinpoint alternate realities where her sons Tommy and Billy still exist. Reaching them is still beyond her abilities, however, so she will need to find another with the power to puncture the barrier between realities.