I have to report that today, on the 43th session of my campaign we had our first character death. Granted, it was not a proper death as it was promptly remedied with a revivify. This looked a tad grim for the characters for a moment though and I got to describe a nice scene where Belet Ummur, the Goddess of Death arrived to collect the slain character's soul, only for him to be called back to life by his comrade.
The characters (a party of four) were on eleventh level. The death was caused by Sphinx of Judgement. (CR 11, the sphinx had somewhat boosted HP, but no flight, as Artran sphinxes do not have wings.) It was part of an encounter with a Solar Bastion Knight (CR 9) and 12 Veterans (CR 3.)
The party had had one encounter previously, but they had taken a short rest after it so were almost in full resources.
This was a CR 23 encounter by 5.0 rules so about 4.5 times the deadly difficulty, and by 5.5 rules a CR 18 encounter, 1.25 times the high difficulty. (The enemies had also four commoners on their side, but I did not count them for the encounter budget, and all of them died of AoE before doing anything useful.)
I think it shows how massively the encounter difficulty maths is changed with the 5.5. update. I made these calculations only after the game, I had just eyeballed it like usually do, and it was supposed to be a really tough fight. I am actually surprised how close to 5.5 recommendations my guessing ended up. But yeah, it seems WotC agrees with my assessment that the encounters can be way harder than they were and have adjusted the maths accordingly.