Warning! SPOILERS for Thor: Love and Thunder.
The MCU's rules for the multiverse have been notably inconsistent, but Thor: Love and Thunder introduces an even more vague concept in the form of Valhalla. The Infinity Saga only relied on a handful of fictional concepts such as superpowers, magic, and the Infinity Stones. They are all outlandish, but they're still clear enough to let viewers enjoy all of the MCU's first 23 installments to the fullest. Once the Infinity Saga ended, the MCU's Phase 4 took an ambitious step further with the debut of the multiverse and the afterlife.
The rules of the MCU multiverse are difficult to define. Loki established that the TVA eliminated all the people who threatened the Sacred Timeline. However, the Avengers were inexplicably spared even though their Time Heist from Avengers: Endgame resulted in Loki's escape with the Tesseract, which in turn spelled the end of the TVA, the Sacred Timeline, and He Who Remains. Variants — who somehow can look identical or completely different from each other — can travel to other universes and go back home peacefully, like the many characters of Spider-Man: No Way Home. Soon after, Doctor Strange in the Multiverse of Madness explained that multiversal visitors cause catastrophic "incursion" events, even though Gamora, Steve Rogers, and Vulture apparently stayed in other universes without a problem. Steve Rogers, for that matter, shouldn't have been able to stay in the past and return to the present without passing through the Quantum Realm. There's also the fact that What If...? gave the Infinity Stones the power to affect the multiverse, even though they're supposed to work exclusively in their home universe.SCREENRANT VIDEO OF THE DAY
Related: Wait, Did Thor: Love & Thunder Bring Back The Soul Stone?
Now, Thor: Love and Thunder does something similar with its idea of the afterlife. Sif, Valkyrie, and Thor talk about Valhalla as if it were a common belief in the MCU. However, no Asgardian ever mentioned it before, and death was treated like a definitive end in all of Thor's previous appearances. Thor also says that admittance to Valhalla is reserved for Asgardians who suffer a brutal, painful death in battle. Yet, Thor's Jane Foster, a human-turned-god, enters Valhalla after peacefully succumbing to cancer well after the fight against Gorr the God Butcher is over. By the same logic, Odin couldn't have entered Valhalla, but the golden sparks that signaled his and Jane Foster's deaths suggest otherwise. Moon Knight had also established that in the MCU, all versions of the afterlife exist inside the same dimension, and that their manifestation depends on each person's beliefs. Even if Mjolnir made Jane Foster an honorary Asgardian, there's no reason why she would end up in Valhalla.
Why The MCU's Valhalla & Multiverse Rules Are Both So Unclear
The multiverse, death, and the afterlife are all very complex concepts, even without the context of the MCU. They have had countless different interpretations throughout the ages, and each storyteller can have an original take on them. Likewise, each MCU writer and director can give them a twist that works for the specific installment they're working on, but that also causes continuity problems for other installments. Settling on one solid definition from the start can limit individual characters and stories. It's more simple to make them malleable ideas that allow other movies to adapt accordingly. For instance, the Sacred Timeline's destruction makes the multiverse possible at every point in time, and Thor: Love and Thunder's vague explanation of Valhalla lets future MCU releases decide whether to bring back every fallen Asgardian or let them rest in peace.
This is probably why Thor: Love and Thunder's second post-credits scene only features Heimdall. Valhalla leaves the door open for Odin, Frigga, and the Warriors Three (and even the original Loki), but it also doesn't force the MCU to bring them back. Meanwhile, Idris Elba's character receives a well-deserved opportunity to return to the MCU. These possibilities suggest Phase 4 is only setting the foundations of the MCU's new saga. Future releases may solve all of the conflicting details regarding the multiverse and Valhalla.