About two months ago, Gabriel Aul, head of Windows Insiders said that if Windows Insiders wants landscape start screen on Windows 10 Mobile they can upvote the suggestion in feedback app or from here.
Now, today someone tipped us two screenshots, showing landscape start screen is under work.
In above screenshots, you can see navigation bar and status bar is in landscape mode, however It seems landscape start screen is in alpha stage and it can be released to Windows Insiders in upcoming months.