Civilization -- is not the same thing to everyone
In addition to concepts of social hierarchy, state, agriculture, domination, separation, cities, technology, etc., written language is often referred to as a characteristic of civilization. If so, then attempting to answer this question through language is perpetuating civilization, rather than living in some fundamentally different way. Color me guilty.
Is there a particular marker in history - no
Is civilization chosen or imposed - both
Does de-civilizing happen socially or individually - both
What does de-civilizing look like - as unique as every individual/community
I think the difficulty with naming things (i.e. civilization, good, bad, trees, whatever) is our confusion, or at least, our forgetfulness about the transitory and unique nature of things. Things are always in a state of change and transformation. Thinking they are independent of one another (civilization/de-civilization, choice/imposition, social/individual), or are stuck in time (a definition of civilization, a particular marker to the end/beginning of it) is an illusion. Simply by virtue of discussion we tend to attempt to fix things in time and space and measure.
When would someone be able to say that civilization has ended? When the S&P500 is again under 100? When electrical blackouts in metropolitan cities in the U.S. are happening every month? When borders between certain countries are no longer maintained by the state? When a particular polar ice cap has completed melted? When a particular number of governments or currencies collapse? When only 20% of the world population is driving a vehicle?
There may be dominant forms of social structure at any given moment, but even within hierarchies there are many differences, and there have always been people living in different ways from the majority. So maybe it’s not so important to be able to label something or determine when it originated or ended. It is our concept of linear time that tells us we must do that. We often learn more from observing and doing than from thinking. I know the most powerful things in my life have come via dreams and emotions and relationships and experiences, fighting and loving, all the beauty and ugliness that is possible at any moment. Words and descriptions can reveal those experiences and feelings and the thoughts about them, they can spark more feelings and actions, but they seem to me to fall short if they become a rigid, static picture, rather than an ever-changing story that we're telling that shapes our lives.
We can get lost in words and numbers and time. How do I live today in this moment, in my relationships with other beings and the planet? What am I resisting letting go of or fighting for? What am I willing to actually experience and observe and feel? These seem to me to be more important questions. But still I can easily find myself talking to someone about “the collapse of civilization” or the “end of money” or some such thing (I don't want any more debt or interest or lawnmowers or McDonalds or borders), until I realize again that I am a unique person telling a story that is part of the collective consciousness, and my focus can move back to a more open state of being, rather than saying "this is how it is or was or will be".
Despite my thoughts about the limitations of language, I often find it quite useful (like on this website), and especially in music and poetry where at least part of the brain turns off for a while. To use Don Henley’s words (despite that they come from a song that appears to be about an individual relationship), I think it sort of sums up my answer to the question posed here about historical markers...
I don't know when I realized the dream was over
Well there was no particular hour, no given day
You know, it didn't go down in flame
There was no final scene, no frozen frame
I just watched it slowly fade away
edited: to more directly answer the question.