For those who have played Assassin's Creed Odyssey, having the choice of choosing between male and female is a massive decision to start with when you begin the game. Not that it overly changes anything in the story; it's just nice to have that option. However, in Assassin's Creed Valhalla, you'll alternate playing as both genders throughout the story in various sections if the default setting is left on.
You can still choose if you wish to play entirely as one or the other; that option is still there for you. Nothing is taken away from you. Feel free to play the game fully as the male or female Eivor. This option is more for people undecided about what to do. You can just "Let the Animus Decide" option, which was revealed in a video clip from IGN. This option is canon.
Darby McDevitt, the narrative Director, explains via Twitter that:
"It means 'Let the Animus decide' is canon," and both characters will appear at various points when you choose this option. So both are canon."
How and when the male and female versions will appear was not fully elaborated on, other than in the game's own mysterious UI description:
"The Animus will represent the stronger FEMALE or MALE memory-stream, depending on its current strength. You may choose another option at any time."
The best thing is that you can repick any of these options at any given point - you are free to choose. Though no doubt it'll be easier just to let the Animus decide who's going to play what section.
Ubisoft seems to have learned their lessons, as, before Odyssey, they were under constant flack for not having any playable characters in this series. Now you have the option to only play as the female Eivor if you so want. Having a choice is fantastic!