My answer was deleted (by '3 users vote' ) and isn't off topic

I'm interested in the eventual, reasons for the deletion of my post and, eventually, if someone could explain them to me, please.

First reason for being deleted (as I am one of the user that voted to delete it) is that the answer is completely wrong. For one, you say and I quote;

Mainly <iostream> is C++ standard library, instead namespace is an advanced command (cpp)

The above statement in your answer is wrong as <iostream> is not the C++ standard library and namespace is not a "command".

Additionally, the deleted answer talks about initialized and formatting but it is not clear what they mean by that.

Also, the statement:

must be included always[...]) to use new and 'special' commands, in ex. using namespace std need .

is wrong and doesn't make sense.

In my opinion, the answer is also unreadable (in addition to being wrong).

I'm reading this answer as, roughly (correcting terminology issues and typos, but trying to preserve meaning):

<iostream> is part of the C++ standard library. namespace is an advanced keyword used as boilerplate setup in almost all C++ code to simplify writing the code. For now, try to see using namespace std¹ like int main; essentially, as boilerplate to copy-paste into a new file. Normally, this is explained after functions are already learned.

It is more important to concern yourself with the #include<...> statements. You will need more than one #include (<iostream> must always be included) to use new and 'special' commands; for example, using namespace std needs <iostream>.²

This is essentially saying to ignore using namespace and just copy-paste it in. The question is what it does; telling someone to ignore it does not answer that question. While we allow frame challenges in answers, this answer is really not challenging the framing, but instead avoiding the question entirely. And assuming the claim is that "almost all c++ codes" use using namespace std, it's also wrong (this is not the only incorrect statement). While just being wrong is usually not a reason to delete an answer, that is not the only one of several issues here.

With respect, your writing style and English usage are quite difficult to read. I did my best to understand it, already knowing the correct answer to the question, and I was still unable to figure out what some of it means after looking at it for quite a while. Please consider using one of the many free grammar-checking tools available online, as well as carefully checking the technical parts for correctness (you wrote "unsing" instead of "using" both times you posted this answer, for example).

¹ It is unclear to me whether the original answer was intended to refer to using namespace std specifically, or using namespace generally. Given the context of the question, I've assumed the former.

² I cannot figure out what this sentence is trying to say. It appears to claim that multiple includes will be needed, but then gives the example of <iostream> again. It also appears to claim that <iostream> must be included to use using namespace std, which is wrong.