Currently, navigation menu orientation is configured at the app level for all users and it cannot be changed by users at runtime.
Let the developer define 2 menu profiles - one for horizontal and one for vertical menu and how they should look like and also set a default menu orientation for the app.
For users, offer 2 radio buttons to switch to vertical or horizontal layout of the menu which would render the pre-configured menu in the chosen orientation.
We are offering users to choose themes at runtime and this menu orientation choice would be something similar for a better UX.