The optional rules for facing are just that. Optional. Without them there is no such thing as facing or outflanking. Rogues dont need to outflank or be behind a character to do suprise attacks either. Another target hostile to their target within 5 feet is enough in 5th ed.
Exactly my point. Because the Facing rules are optional, the base rules have no facing. Thus, there is no "behind" a target. In 5e combatants are constantly moving - and importantly, turning - around. While supermarket you isn't able to literally see out of the back of your head, 5e characters in 6 seconds will be looking around and aware of all of their (visible and audible) surroundings, including behind them.
True, I was mistaken about the requiring a hide check to get advantage. As long as you're unseen you still get the advantage. But this doesn't change the fact that simply standing behind an enemy doesn't grant advantage. Because there is no "behind" in base 5e rules. This is part of the reason why stealth in BG3 is so cheesy: enemies cannot detect a stealthed player unless the player crosses their sight cone, whereas in 5e you shouldn't be able to hide in plain sight.
