I mean, when all is said and done, 5E can still be a pretty lethal game. I wouldn't consider it any softer than 3/3.5E.
Power Word: Kill is in particular is far more lethal in 5E than 3.5E - both auto-kills at 100 hp, but 5E is a lower HP game compared to 3.5E (where characters have far higher CON stats). Also, in 3.5E spell resistance can save you from it. Disintegrate does packs less of a punch, but it can also land far more in 5E compared to 3.5E due to the way saves work.
For example, a 5E level 20 wizard with 16 CON, will only have an average 98-ish hp (6+ 19*3.5 + 20*3). That is still in range of a OHK by Power Word: Kill.
They are also in range of a OHK by an upcasted level 9 disintegrate (19d6+40 = average 106.5 damage). A single casting of Meteor Swarm (40*3.5 = 140 avg. damage) will also put them down.
In fact, a level 20 Great Weapon Master (GWM) Fighter can action surge and make 8 attacks @ 22 average damage (2*3.5 + 5 + 10) x8 = 176 total avg. damage)), which will also one-round kill that level 20 Wizard
A GWM bog standard Paladin can smite twice @ 49 damage (2*3.5 + 5 + 10 + 4.5 + 22.5), which is just enough to one round knock out that same level 20 Wizard.
