Samsung has added the option to enable screen rotation for the lock screen as well. The same option can be found on other devices running One UI 1.1, which is pretty much every Galaxy phone that runs Android Pie out of the box.
And it’s not just the lock screen that can be used in landscape mode — you can also enable or disable screen rotation for the calling screen.
Here’s how you do it: On your phone, swipe down from the top of the display twice to expand the notification shade to see all quick toggles, tap the text that says Auto rotate (or Portrait or Landscape, in case auto rotate is off), and then enable the toggle switches next to Lock screen and Voice call screen. For a visual guide, just see the GIF animation below
Once you’re done, your phone will show you the lock screen and/or voice call screen in landscape mode when you have auto rotate enabled. There’s one caveat, though. In landscape mode, the lock screen does not show the content of notifications, only the icons for the apps that have pending notifications. You can, however, tap any of those icons to expand the notification shade without unlocking the device.