The thread was unfortunately titled and it can't be changed after it is created.
The problem we are addressing is that landscape becomes unavailable in ANY application, on both iPhone 6 and 6+ and so this is not just about the home screen mode on the 6+. There appears to be an OS bug that prevents rotation to landscape orientation of ALL apps. This problem has been reported as related to restoring from iCloud backup or use of the Zoomed display feature, or both. Not really confirmed at this point and it is not readily reproducible, but many folks have experienced it.
I found that opening the calculator and rotating phone would demonstrate the problem (calculator doesn't rotate to landscape). Checking lock rotation icon in control center shows it isn't locked. Restart phone. Open calculator, still won't rotate. But when opening control center, I see the rotation lock icon is set. Click to unlock and all is well. Problem has not recurred for me despite several restarts.
I hope this helps.
Aloha