The sound [ng] in the Japanese language.
I have read in a textbook that the sound [ng] in the modern Japanese language is vanishing. Many young people prefer to pronounce the sound [g] despite of its position in a word. Is it truth?

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Answer

I think it's the same as "а" "о" issue in Russian (some people pronounce бАльшой, some бОльшой).
Even if the "ng" sound is vanishing...it's not going to disappear just yet.
Which textbooks you're referring to?

Wikipedia says:
    A. majority of speakers uses either [ŋ] or [ɡ] in free variation:  /kaɡu/ → [kaŋu͍] or [kaɡu͍] kagu 家具 'furniture'
    B. minority of speakers consistently uses [ŋ]:  /kaɡu/ → [kaŋu͍]   (but not *[kaɡu͍])
    C. smaller minority of speakers in Kantō consistently uses [ɡ]:[3]  /kaɡu/ → [kaɡu͍]   (but not *[kaŋu͍])

I don't think the information is wrong. Personally, I try to follow the B pattern and use "ŋ" consistently.