A deleted scene from Avengers: Endgame supports the theory that Thanos is actually still alive. Marvel Studios has celebrated the launch of Disney+ with the release of a number of key deleted scenes for Avengers: Endgame. They included an alternate version of Black Widow's death, and of course the much-anticipated Soul World scene featuring Tony Stark and his daughter, Morgan.
One of the more interesting deleted scenes is an alternate version of the Hulk's conversation with the Ancient One. The temporal mechanics are fairly similar, with the Ancient One explaining that the removal of an Infinity Stone creates a new timeline. In this version, though, the Ancient One has far more advanced knowledge of the future; contradicting the events of Doctor Strange, she's been able to use the Time Stone to see beyond the moment of her own death. It leads to a fascinating discussion in which the Ancient One drops a bombshell: she knows how the snap will work.
According to the Ancient One, the snap didn't actually kill anybody at all. She suggests that there's an important functional difference between killing someone, and doing what Thanos did, which she describes as willing people out of existence. Death is final, the snap is not; resurrection is impossible even with the power of the Infinity Stones, but the snap can be undone. It's a fascinating comment, neatly explaining why snap victims could be restored but Black Widow couldn't be brought back. Oddly enough, it's the first time Marvel has ever explained how the snap worked at all. And it carries profound implications for the MCU, because it strongly supports a theory that Thanos is still alive.
Avengers: Infinity War and Avengers: Endgame both ended with a snap. In one, Thanos snapped his fingers and erased half the life in the universe; in the other, Tony Stark snapped his fingers and erased Thanos and his army. If the first one didn't really kill, then it stands to reason that neither did the other. A good analogy is that the snaps essentially function like the transporters in Star Trek, converting matter from one state to another, leaving a record in some kind of "buffer" plane of existence. If Thanos' victims were left in a kind of "cosmic cold storage," thus able to be restored, then that is now the Mad Titan's fate as well.
If this is indeed the case, then there's no reason to assume Thanos is gone for good from the MCU. It's quite possible that, somewhere in the depths of space, some alien beings managed to work out what had happened - and even reverse it. Doctor Strange wouldn't be aware of this, having focused on events on Earth and Titan when he explored 14 million futures, so he wouldn't have known there were other ways of accessing this buffer plane of existence. Marvel have quietly given themselves a get-out clause should they ever wish to bring Thanos back.