Is there a way to put all apps in landscape mode, including the Home Screen?

Most apps support landscape mode today, just as the iPhone 7 Plus also already supports the Home Screen in landscape mode.

Make sure you have not activated Screen Rotation Lock.

Pull the Control Center up from the bottom - look for the lock with the arrow around.

If it is activated as in the picture, turn it off.

dfh ~ No, the reason being to ensure widgets can remain in place as organised by the user in vertical mode:

Use widgets on your iPhone and iPod touch - Apple Support

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