Are sentences like:
- OK!
- Good morning!
- Bye!
- Happy New Year!
- Sorry!
- etc.
also grammatically defective and acceptable only in a poetic context?
They're grammatically defective if you want to call them sentences. But I'm not making any value judgements about grammatically defective phrases. The question was "How's that sentence?" and I reasonably pointed out that it was missing a verb.
I remember one my colleague, after passing his PhD studies in USA, started to maintain conversations by sentences:
- Right! (responding to any phrases)
- Agreed!
- Good heavens!
- many others
Should people start avoiding such an "education"?
Have he got poetry education instead of intended technical one?
You're assuming that people speak in sentences. There's nothing wrong with any of those utterances as communication, but that doesn't make them sentences.