I'm not a native speaker but I think I can help you. 안녕 is used as the "hello" or "goodbye" in Korean. In Hanja, or the Chinese character counterpart of it, it has another meaning. 安寧 is the Hanja counterpart of it and it has the meaning of peace, safety, or tranquility (same for Chinese and Japanese). The 안녕하세요 expression came from the word 안녕하다 which means to be well, so they say it when they leave. But as the time goes by, the use of Hanja gradually declined due to the use of Hangul. So the meaning of it became vague/ambiguous and as a consequence, some Koreans does not know the real meaning or the root word of modern vocabulary. So, I could say the meaning of it became obsolete since they have another term for peace which is 평화/平和 (same for Chinese and Japanese). Nevertheless, if you want to use 안녕 as peace, use 安寧 instead.

안녕, as a noun, means "peace, stability" This usage is almost obsolete nowadays. ex) 그는 떠오르는 태양을 바라보며 가족의 건강과 안녕을 빌었다 As he watched the rising sun, he prayed that his family live in peace and good health. 안녕, as an interjection, means "hello, hi"

I'm not a native. It means "peace" based on its origin and so became part of the greeting phrase 안녕하세요?/안녕하십니까? (Are you at peace?). However, when talking about peace, 안녕 is not used. There is a different word when discussing peace. Today, 안녕 only means peace in the context of the greeting but it just means "hello/bye". In English, there are certain words that we use only in the context of a set phrase or saying, but we wouldn't use in everyday life. The same is true of 안녕. 안녕히 would thus mean peacefully. It is used to say good night.

I'm not a native speaker but I think I can help you. 안녕 is used as the "hello" or "goodbye" in Korean. In Hanja, or the Chinese character counterpart of it, it has another meaning. 安寧 is the Hanja counterpart of it and it has the meaning of peace, safety, or tranquility (same for Chinese and Japanese). The 안녕하세요 expression came from the word 안녕하다 which means to be well, so they say it when they leave. But as the time goes by, the use of Hanja gradually declined due to the use of Hangul. So the meaning of it became vague/ambiguous and as a consequence, some Koreans does not know the real meaning or the root word of modern vocabulary. So, I could say the meaning of it became obsolete since they have another term for peace which is 평화/平和 (same for Chinese and Japanese). Nevertheless, if you want to use 안녕 as peace, use 安寧 instead.