You ask for "gone and why" (in question and comments) but post an answer "reduced effort, without specifics", so you didn't answer your own question.
You make a comment: "The list of users is what the question asks for. The links are supplemental information that is interesting but not not essential. I don't know how to react to the suggestion that don't know how to answer my own question because I was polite to someone who turned out not to be understanding what was asked.".
There was and is not any such suggestion, what I wrote was clear.
You don't understand that you changed the question, hedged the answer, and made a comment which changed, rather than explained, what you were asking for: "@Rob: "Where's the SEDE link?" doesn't answer the question of why. – rockwalrus-stop harming Monica 8 hours ago".
One way to react is: don't backpeddle, suggest you are polite, and that others don't understand English. Your account really has little reputation on other sites, and all the reputation you gained here was derived from slamming. We can explain this to you but we can't understand it for you. We don't need you to explain how you were caught.
As to your original question, here's why it's so difficult, unless people come back to answer you.
SEDE: Active on Stack Overflow:
Since January 2018 until last Sunday (for Stack Overflow only) is plotted above. There's a small upward trend but month to month variance is quite large, it's very difficult to say one particular person's contributions are not replaced by one to several others - and that's the bottom line.
True, the chart doesn't show "who" (but it could) and especially not why; with that many comings and goings the reasons for leaving (and coming back) are going to be quite varied.
Three? answers saying: "Vote if you are annoyed by X" could work, but isn't allowed.
Does a list of users that have announced they are suspending activity exist?
The huge list would need to be constantly updated, see the chart above.
Overall things do improve (for the most part) year to year, there will always be quite a few that preferred "the old days" replaced by many complaining that they don't understand the new ways, only to be told ...