I found specially in spoken english speaker omits Is and are. Is there any specific grammar rule for this? E.g.
Are you ok? And You ok?
You are always busy. And You always busy.
You are writing this since 2 yrs. And You writing this since 2 yrs.
The only time I can think this common is with questions asked directly to a person or persons,
You coming? (Are you coming?)
We ready? (Are we ready?)
We all good? (either "Are all of us 'good'?" or "Are you and I completely OK with each other?")
It's colloquial and informal, appropriate to use in casual conversation. I would say it's not appropriate to use in business meetings or the courtroom, but both are much less formal situations than they used to be (at least in the United States).
You always busy
might be accepted in AAVE, where the copula can be dropped in the present tense and first or second person. But it is not standard American English, so I wouldn't recommend using to an English learner.
is pretty common in informal situations. I suspect this has been adopted from AAVE by other Americans.
You are writing this since 2 years
You writing this since 2 years
are both non-standard. I'd say "You've been writing this for 2 years." Notice that in your first two examples, "are" is the copula, while in this example it's part of the present progressive of to write, so you shouldn't expect it to behave the same.