@Mangareader
As far as I'm aware, the only main games set before Altaïr/Ezio are Origins and Odyssey.
In Origins, Bayek receives the hidden blade and uses it to assassinate a target.
He struggles to kill his target, however, and ends up losing his finger to his own blade. As a result he has to flee the scene while bleeding profusely.
This implies that you can use the hidden blade without cutting your finger, but it is safer to cut it off beforehand.
In Odyssey, there is no hidden blade. Kassandra/Alexios have attached the head of a broken spear to their belt and uses it to stab their prey instead.
The whole point of removing the ring finger is so the assassin can use the hidden blade on the inside (where the finger would obstruct the blade's path) without accidentally losing that finger in the middle of combat or during an assassination. So if Valhalla has the assassin wearing the "hidden" blade on the outside, there's really no need for them to cut off their finger.
@Nolonar the cutting off of ones finger has two roles. One was so that Assassins could use the blade, but in AC2 Da Vinci upgraded it so that it was no longer necessary. The second reason was that it was a test of loyalty for the Levantine Assassins. The only character we’ve played who purposely cut off their finger was Altair. Bayek started the tradition but it was an accident.
Eivor puts it on the top of his arm for a different reason though. Ubisoft says it’s due to cultural tradition for the Norse, but I bet it’s something like “it’s not cool to hide a weapon bro” Valhalla takes place before AC1 so it’s possible Eivor might still take up the tradition of cutting off his/her finger, even if it’s not a requirement to use the blade.
Dang when the trailer of Valhalla came out I just finished watching the anime, and then I just binge read the manga. I always wanted to see this crossover, but never thought they'd actually do it. Too bad Valhalla's set more that a hundred years before vinland saga, but it’s still soo goddamn cool to see this