This was a legacy element from before we had mobile applications and was removed as part of the web access revamp in 5.0. If you are using a non-iOS or Android device note that the default web view is now more responsive and should be more usable on your mobile device than the previous /mobile page itself was.
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Maybe is more responsive but more usable definitely not... everything is so small on the new one on my note4... (I can't upload images because "The file is too big, maximum allowed size is 256 KiB."...)