I dunno, I'm not sure I agree with that. I find that when I use "deadly" encounters, I can really push the party. I don't follow the DMG guidelines on how many "encounters per day" or anything like that (the party should know when it needs to back out and take a rest, especially with all their experience). The only thing I look at is how the encounter rates after I put it together. I don't care how may "deadly encounters" I use in a day, and most of my encounters probably end up on the hard side. Now, the party is just about to hit 6th level (after next session) and I have yet to kill anyone, but I have had several of them down and making death saves at one point or another (and, to their credit, they are VERY seasoned players).
Still, it is true, these games will likely never be as deadly as they were back in the 80ies. It's just the perception of the game nowadays. Players are more demanding and systems like 4e REALLY neutered the DM and created players with a sense of self-entitlement. 5e has put the power back in the hands of the DM.
I am curious as to what you do in your game to make it more deadly.