When you post a novel work, you can change the display default from horizontal to vertical.
▶On desktop
▶On mobile
▶On the app
On desktop
Unfurl the Advanced settings menu on the posting page.
Under the Viewer options, enable "Display text vertically by default".
On mobile
Unfurl the Other settings menu and then enable "Display text vertically by default".
On the app
Currently, you cannot select vertical display on the app.