Those encounter difficulties were thresholds, not hard numbers. At 1st level medium isn't 50. It's 50-74. Deadly isn't 100. It's 100+. So a deadly encounter by the 5e rules could easily result in a TPK. This is especially true since encounters were so easy the DM had to go past the thresholds by a significant margin to actually challenge a decent group.
I don't think they will often result in a TPK unless the DM is purposely trying to kill a party.
You say it can easily result in a TPK, how many of those have you seen in play and more importantly how many that you saw were due to a custom monster more powerful than the DM thought and not one of the other things I mentioned?
I can say in well over 1000 sessions I have never seen a TPK in play in 5E because the DM made an overtuned custom monster
That means proficiency with encounter creation and the ability to recognize when you are approaching that razor thin line are a must if the DM wants to challenge a decent group and not TPK them.
We are not talking about encounter creation, but rather monster creation. Those are two different things and we are presuming that said DM does understand how to create encounters of the appropriate difficulty if the monsters are appropriately speced.
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