Can a junction table (join table) also be used for a one-to-many relationship?

There is not any reason why a junction table couldn't be used for a one-to-many relationship. The question is usually one of performance. Why make the database join an additional table when it is unnecessary?

Yes, it is still possible to store and enforce one-to-many relationship in a junction table.

In your example you are not enforcing any constraints on the UserOrders junction table, so a single order can belong to two users (assuming that's incorrect). To enforce that you could make OrderKey be the primary key of the UserOrders junction table (or have a unique constraint on that column). Technically that will just become a many-to-one relationship between UserOrders and Users, while having one-to-one relationship between Orders and UserOrders.

I can only think about one reason for designing the many-to-one relationship using junction table - if you plan to allow the many-to-many relationship in future and don't want to deal with data migration. But in the mean time you will pay the cost of storing and joining with additional table.