Yes I've tried and we cant.
Yggdrasil, or the World-Tree, is an immense sacred tree in Norse lore. The enormous array of glowing branches that can be seen very high in the sky during clear weather is a part of the World-Tree. According to the in-game lore, this tree was once anchored to Valheim, as well as 9 other worlds, before Odin severed the boughs that connected Valheim to it after designating the world as a prison for his foes.
https://valheim.fandom.com/wiki/Yggdrasil

I don't think there's any reason to fly up to the branch. I'm more interested in what happens if you sail in the direction of the branch and find the place where the branch meets the water. Or is there like a big waterfall that serves as the absolute boundary of the world?

Maybe in a later update, a true endgame can be provided in which you kill the last of the Forsaken, and are granted the right to leave Valheim and go to Valhalla, and the exit can be found by following the Yggdrasil branch.

If you use the console and just fly up there. You will fall through it. It looks cool though.

I flew over there once to check it out, it has no physical interaction, you just fall through it.

skyboxes can be quite cruel :D

I'm not going to lie it would be an awesome final biome though, like you'd have to battle through all the bosses again only harder and you made your way to the truck or the tip of a branch where you would fight Thor or even Odin himself.

It’s supposed to be the branches of Yggdrasil that Odin chopped off to sever Valheim from its connections to the other Nine realms and, currently at least, is nothing more than a cool touch to the skybox.

Early on I speculated that there might be a portal after the eighth or ninth boss in the finished game that might take you up on top of the tree for a final "biome in the sky". Some people have used the -console to activate fly mode, and they have flown up to check it out. Turns out currently you just pass through it, it isn't a solid object as of yet.

So i guess to directly answer your question : Yes.