Not sure what to say when I read things like this. I have never had difficulty challenging PCs at any level. The last fight for my previous campaign I came close to perma-killing one PC, had others in single digits at times. They were all level 20 and because of other constraints on time (one player was going to be moving) I hadn't worn them down as much as I wanted. Game or two before, I almost killed 2 PCs in what i thought was going to be a moderately difficult fight.
I have no magic formula, I don't fudge rolls one way or another, theoretically we're using basically the same rules, it wasn't a triple deadly fight according to my calculations. I do increase the number of legendary saves and actions to number if players in the party minus one. I also adjust monsters here and there.
The good news is that they acknowledge the issue and are taking steps including more high level monsters. But challenging high level PCs has been possible in every edition of the game.
I have character deaths. I have close calls.
I think you overlooked the parts of my comment that I considered necessary to the point, I probably should've bolded them or expanded on them
1. "Appropriate-CR" monsters are a pushover for high level characters. The Pit Fiends, generals of hell, have +14 to hit; one character had 35 AC, the other was in the high-20s. Even if they were hit, they had lots of ways to negate or avoid those hits. The two characters, between them, could easily weather and restore themselves from anything the pit fiend or devils could actually do to them. Those two characters probably could've handled an Ancient Blue without any real chance of danger. TWO characters, not even a party of four :')
2. that it's difficult to challenge high-level characters
without turning your setting on its head. This is setting-dependent I guess, and it really just reinforces the other bits I said: I don't like running high level 5e, because to appropriately challenge a tier 4 party you need to be essentially exhausting your setting of its most powerful beings.
In 5e (as compared to other editions I've run), the PC power has gone up, and the monster power has come down. Even through the 5e's lifetime, you can see that monsters have become easier for their CR. Look at the CR 30 Tiamat from Rise of Tiamat, vs the CR30 Aspect of Tiamat from Fizban's. The Aspect is a pushover, but it's still CR30. The "Great Wyrms" from the same book, are likewise pushovers for their CR. It isn't just that book, you can go look at the Vecna adventure to see more examples of "the most deadly foes" that are basically just feel-good beat'em-ups.