That is a long discussion, but the short version:
In optimized parties: Often, yes. The difference between optimized and non-optimized at 20th level can be extreme. The most powerful optimized parties can cruise through a lot of CRs up to 26.
However, my point, from a year ago, was apparently not clear - It isn't that you can't challenge a group of PCs lives in every encounter at 20th level. It is that you shouldn't. They're the greatest of epic heroes. If every moment is a life and death struggle, they do not feel that way. Instead, make them feel like the heroes of legend by giving them challenges that don't threaten their lives, but threaten the lives of those that believe in them - at least most of the time. There should still be encounters that challenge their lives at times, but most encounters should present other types of challenges. It is part of the evolution of the game and the evolution of the stories you tell... D&D is an RPG - a role playing game. Characters play a role in a story. 20th level characters are at the end of their third trilogy of novels as PCs - they've earned their stripes and you're about to retire them and let a new generation of heroes rise in their place.... They need to feel like the pinnacle of the game - they've earned it.