Apple knows what's good for you - how dare you criticizing their benevolent parental guidance? 😁
Jokes aside, I'm also a little bit puzzled that there's no option to choose between the "toggle on-off" setting in landscape mode and the fade out one as in portrait In Safari's settings. But there's actually some kind of workaround I'm now using most of the time in landscape mode:
When tapping the URL address bar your bookmarks will also show above the virtual keyboard as they do when Safari doesn't have any websites opened. As soon as one taps into that bookmark window to scroll or directly click a bookmark the keyboard disappears again. It may not be a perfect method, especially when bookmarks aren't organized in folders, because the top level bookmarks are displayed as rather largish icons. But those contained in a folder will actually display as a drop-down list (can be scrolled when having a large number of entries) similar to the normal bookmarks list. Once a bookmark has been clicked everything fades out and you're taken to the selected site. As my bookmarks are almost exclusively in folders such as "News", "Sports", "Tech" etc. I've come to appreciate this kind of bookmark navigation much more than through the standard bookmarks menu with its annoying behavior in landscape mode.