...1.What is the difference between "was published" and "has been published"
Was published: past tense, passive voice.
Probably used to describe something no longer published.
Has been published: present perfect tense, passive voice.
Probably used to describe something which is still published to this day: an ongoing publication.
2.What is the difference between "It is as you've heard" and "It is as you heard". Or both are correct. for "you've heard" we use this because it's
Well, one motive for choosing the past tense would be if the time of hearing was specified:
It is as you heard last night.
It is as you have heard last night X
We couldn't use the perfect tense in such a context.
3.If possible, please explain why Americans use past simple instead of present perfect for example " Did you have lunch"?
......... I'm not sure, but I am sure debates/arguments on such issues have already occurred on this forum. I am English and I would use the present perfect (have + had).