Google n-gram indicates that coucher de soleil has been gradually gaining ground against coucher du soleil over the long term, and even over just the last 20 years.
I didn't realize both were permissible before today. Can someone elaborate on the difference in meaning or tone, if there is any? It seems profoundly subtle a difference of pronunciation to me, I would have immense trouble even telling the difference between the two spoken at speed.
A few thoughts based on my experience relative to that question and a little help from the French academic dictionary ("de" for "spectacle")
The question is one of standard language versus language that is coming into being without true motivation, without the real need for it. For simplicity, it's best to keep to a plain, standard, correspondence between context and form; when the sunset is spoken of as the point in time of the simple disappearance of the sun below the horizon the preposition ''du'' should be used: le coucher du soleil, un coucher du soleil, les couchers du soleil.
When the word "sunset" is considered as a spectacle as in "I had never seen such a beautiful sunset" one should use "coucher de soleil" (specifically in translation of this example: un coucher de soleil). This particular idiom containing "de" connotes the spectacle of colours in the sky a fairly long time before the sun disappears. The use of "du" instead of "de" is a matter of idiomaticity which is gradually eroding.
The subtle difference of pronunciation that seems to be apprehensible has no reality; there can be no ambiguity between "du" and "de" if those words are pronounced normally: one should never pronounce them so fast that they should be difficult to differentiate and they would have to be pronounced awfully fast for that to be the case: only slipshod pronunciation is to be held responsible for that.
