The single big timeline is confirmed in Metal, from the birth of Multiverse and Perpetua's imprisonment until Rebirth, Perpetua has been whispering to the movers of DCU like Anti-Monitor, Parallax, Alexander Luthor, and Zoom to cause Crisis to produce large enough Crisis Energy to eventually release her.
Before that, I thought that Rebirth is the same as New 52 but with some alteration, its own continuity, separate from the others, and the Crisis never happened in this continuity's history, but then Bendis Young Justice revealed that all Crisis is canon. At that time I didn't know why and how. It turned out to be that the Crisis happened before they became New 52 and Rebirth, and all of DC's main canon happened in one single timeline.
The first hint was The Button. When Barry ran back in time, he managed to reach the Flashpoint universe, even though the Flashpoint universe was supposed to be gone with the creation of New 52. During the run-through time, Barry and Bruce saw the Silver Age versions of themselves. They thought they're alternate versions of them, but Zoom said, no, these are all you, not alternate versions. Then he's taken by Manhattan before he can spoil anything anymore. (I may mix up some details here on which character does what but the important thing is, they concluded that the Silver Age and Rebirth are the same people but changed)
Another hint was Alternate Future Evil Tim, who knew that Cass and Steph were Batgirls once, but that memory has been forgotten, and that fact has been altered.
The concept of past characters evolving (I used the word transforming in the previous post) has been there since Convergence at least. At the end of Convergence, it is said that the Post Crisis characters evolved to the New 52 with the exception of characters who escaped it. Lois, Clark, and Jon. (the nature of the Superfamily is later changed but the concept of Evolving Characters were there)
Actually, Johns probably already used the concept of evolving character since Flashpoint, because at the end of Flashpoint, Barry handed Thomas Wayne's letter to Bruce, and that letter still exists in New 52, and later Rebirth. Even though Barry thought everything has returned to normal, that he's returned to the Pre-Flashpoint universe, he didn't. He returned to the New 52 universe, and he became New 52 Barry. He entered Flashpoint as Pre-Flashpoint Barry but came out of it as New 52 Barry with all of his New 52 back story.
Of course, the idea that they're all the same character in a single timeline is supported again once Gold Wonder Woman in Metal unlocked the Truth, that is, the memory that they all have past versions before they evolved into the current version. When Zatanna did it to Tim, I didn't get it. It was only after Metal revealed the single long timeline that I get it.
This concept still stands now in Infinite Frontier. After Metal, Barry said that they all will remember moments from their past versions in flashes.
Doomsday Clock also treats it this way. Every time a Crisis happened and the multiverse is changed, Earth-0 and its inhabitants became a new version, while the old versions that no longer exist were archived in the Metaverse, as a way to preserve each version of the Multiverse. Their past selves of New 52 are archived in Earth-52, the Pre-Crisis in Earth-1985, meanwhile, Earth-0 move on. (I'm still not sure on how the archiving works, but for now I take it as the Metaverse copied and pasted a past version to a different Earth before Earth-0 evolve to a new version)
So Death Metal, Doomsday Clock, Bendis' Young Justice, Snyder's Justice League, The Button, and Convergence. The rest I conclude based on what's been mentioned in those arcs.