Not only is it not real now, but if I'm honest the tradition of optimization in D&D has largely been about avoiding the 4-8 encounter day (depending on edition) for a long time. Rope trick pocket dimension shenanigans, 5 minute utility spamming in 4E, and so on. It's incentivized by the system at the same time as being balanced against it, so if I were being generous I'd say it was a central tension of the game. If I were less generous I'd say it was a flaw.
Part of why we still have a degree of linear fighters quadratic wizards in 5E is because despite the reigning in of spell slots, specific broken spells and non-concentration effects, casters shrimply know they'll rarely go past 2 encounters before being able to bail or rest. It effects the balance of the Luck feat, sorcery points, and things of that nature as well. It means we rarely have to think seriously about hit dice beyond 4th or 5th level. Prior to Witchlight, WotC seemed to still be endorsing "the long day" in 1st party content, but I think this represents them finally moving on how people actually run the game.
It's one of very few things about 5E that would actually necessitate an edition change to fundamentally fix, though it's obviously not going to cause that by itself since people don't care about it enough to disengage. D&D is (in-character) safer now, and a lot of players expect dungeons to be, in the words of Cookie Monster, a sometimes food. I think we'll see an officially unlabeled but fan-recognized intra-edition divergence between these two playstyles, Long Day and Short Day. As far as I'm concerned, it's for the best: players and DMs will have a clearer idea of how to find the kind of game they most enjoy.