I really struggled to find documentation this a while back, but it does exist on the NavigationBuilder property. It's now called "AutoInclude". Documentation here :
It's used like so :
builder.Navigation(x => x.Customer).AutoInclude();
A guide on it here incase you need to opt out too : https://dotnetcoretutorials.com/2021/03/07/eager-load-navigation-properties-by-default-in-ef-core/