Agreed, since "peacefulness" is a relative term, which requires prior experience of something "non peaceful" to compare it to. Which a dead brain would not have memory of the painful times to compare it to. And heck, if death wasn't somehow permanent, and you *only* experienced peacefulness forever afterwards, it wouldn't be felt as peaceful, but as intensely boring and unchanging! Forever!
...But that's all a moot point anyways, since we don't get to experience what if "feels" like to be anything in any state once we're dead anyways.. Feeling requires the organ responsible for receiving and translating the sensory and electrical input to still be functioning. Death means that isn't happening.
Which makes me sad, because all of these people longing for peace (even I have longed for "peace" in death, before I actually thought about it and realized what actually will happen) are literally just going to be experiencing their final moments of catching the bus, and then nothing... It'll peak with the most uncomfortable part of the process—but no peace ever reached, lol... Literally.

! Don't CTB for the sake of peace; if I were going to do it, it'd be because of pain and *absolute genuine hopelessness*, which is rare, but does exist.
Ah well. At least there's other reasons to CTB than just peace. Like to avoid massive amounts of pain and suffering (cancer, death of entire family, ancient times where they would CTB to avoid slaughter and torture from raiders/enemy forces, etc etc). Which even that can't be felt after death either (the escape from awful circumstances). It just sucks that one can't experience that fact that it's over, and that the "bad stuff being over" has to be earned through fear and misery from dying (happens in all deaths, to some extent)—and that's the whole experience. No peace or satisfaction ever felt! Lol. Reality is absurd. In so, so many ways.
At least we (well, most of us) get to avoid going out the way pretty much all animals do (and practically all humans used to, up until the last 100 years or so): getting torn into pieces, or being eaten alive by packs of predators, or getting squashed by something way bigger than oneself/etc, etc, etc...
Hell, even for humans before 100 years ago, the best death you could expect naturally (can't really count heart attacks—sudden death from heart problems becomes super rare the further back in time you go, because most people died young while their hearts were still healthy) was probably cancer, infection or disease. Unless you consider being viciously murdered by humans or other animals as being better than the other options...?
I mean I guess dying as an infant doesn't *seem* as bad as these other options either. But that can't be verified. Just because we don't remember our early years doesn't mean we don't feel everything we do when we're older. lol. Like OBVIOUSLY. But maybe it wouldn't be as bad (hopefully) since it happened CONSTANTLY. Well over 50% of kids didn't make it to age 4 or 5. So effectively, 50% (number might be higher even, actually) of all humans died before age 4-5.
So I'd say even with everything being relative, I'd still pick the final moments of suffering nowadays, from the modern forms of CTB'ing (or most forms of death happening in hospitals, for sure) VASTLY, over *any* of the ways humans and animals died in the past; being able to feel peace after the fact or not be damned!
Edit: sorry this post was so long and off topic, lol... Super overtired. Time to head to bed

...
Double edit: I guess after thinking about it more, there absolutely WAS a peaceful way to go in ancient times, that would be fairly easy to pull off: starvation!
All you'd have to do is not hunt/scavenge, and you'd be set! ...As long as you had the strength to fight off predators trying to make use of your newly weakened body, lol...
And I doubt if you were in a group they'd be super happy about someone being dead weight and not contributing. Death was so common then, that I have no doubt they'd be easily willing to let you starve if you were suicidal.
Weirdly, suicide is super duper rare in ancient times, or in jungle tribes of hunter gatherers. Depression doesn't really exist in those contexts. It's a purely modern day civilized world phenomena. So I doubt anyone was willingly starving themselves.