I'm going to answer your principal question, comment on your approach to revising your questions on english.stackexchange.com and finally attempt to address each of your supplementary questions.
Primary question
Is technical copywriting jargon or style?
Technical copywriting is a style of writing that employs technical terms. People unfamiliar with those technical terms would regard them as jargon.
The word jargon is not always used in an insulting way, it is sometimes used as a complaint by people who feel the written material uses technical terms which are inappropriate for it's audience.
Revisions of questions
As I write this, you question appears as a rather long, slightly rambling, collection of complaints, supplementary material and additional questions. As a copywriter you will be aware that this is not the best way to communicate a complicated subject to your general audience here.
I believe that the facility on english.stackexchange for editing questions is there to enable you (and others) to gradually improve the question. I may be wrong but I believe improvement would best be accomplished, not by periodically extending the text with new material, but by rewriting to produce a clear succinct expression of the question.
Supplementary questions
Technical writing is jargon using incorrect English words.
As others have said, jargon isn't incorrect English. It is only a specialised vocabulary used by a limited group of people. Every profession has their own jargon. This is nothing to be ashamed of. It is only when we use that private vocabulary to communicate with people outside the group that we are making an error.
definitions of "prepend" in internet, all with inserted derogatory remarks ... like 'jargon'
Describing a specialised vocabulary as jargon is not especially derogatory. It is usually faintly disparaging but not always. In this reply I am mainly using jargon to mean specialised vocabulary used by a small group of people.
Is Technical Writing jargon or writing-style ...
You restate your main question - see above for my answer.
However I note that in various places you say
- Technical copywriting
- Technical writing
- Copywriting
I am assuming you mean the same thing in each case.
and the branch of correctly used English?
Technical copywiting can be regarded as a branch of English, a subset of English writing, that is a legitimate use of English. When done well, the appropriate audience should regard it as a correct use of English.
Would I better avoid to refer to it as English at all,
I see no harm in referring to a subset of English as English. All of us do. If it involves technical terms you might refer to it as technical English
If I write using the words which are not English WORDS, do I write in English?
Yes. English is still English even when it uses the occasional non-English words where necessary. If I say I visited München, I am still writing English - though some might wonder why I chose not to use an anglicized form of the place-name. If you carry this too far, you might end up using a mixture of languages.
The term Copywriting "refers to writing ... non-technical material". Where is here a "jargon ... nearly impossible for the average person to decipher"?
Are we still referring to what you elsewhere call "Technical copyrighting" or are you making a distinction?
In my inexpert view:
Copywriting for a general audience should not contain unexplained jargon.
Technical copywriting (for a technical audience) should make appropriate use of a technical vocabulary that a general audience might regard as jargon.
What is unclear in "prepend" and to whom?
So far as I know prepend isn't in general use amongst English speakers - I think some of the other answers may have shown this - I'll update this part of my answer with some statistics later, if I can find some.
Why isn't busyness letter or step-by-step instructions a formal writing?
Business letters and step-by-step instructions can be formal writing. You could write them in an informal way but this would often be inappropriate.
Should it be be understood that insulting obscenities known to everybody are not jargon [but] full members of "correct" English ... [whereas] understandable "technical" ubiquitous words are out?.
No it should not. Some insulting obscenities might be both jargon and full members of English. English jargon words are as much members of the English vocabulary as English obscene words. As discussed elsewhere, there is no single authoritative nor prescriptive authority for what is English. I think you may be focussing on the wrong thing here. At issue is whether it is appropriate to use certain words in certain types of writing - not whether they are English words.
Do you know anybody who ... does NOT know the words RAM or CPU?
Yes. Members of my family have complained to me saying "why should I need to know this? I just want to send an email to my sister!"
Do you know anybody who ... knows, without consulting with dictionaries, all "correct" words of a language?
Yes. JRR Tolkein knew all the words of the languages he invented. I doubt anyone could recite the million or so words that some say are in English. I don't think you can draw any conclusions from these two facts - certainly none that seem relevant to your main question.
May I ask [people] to [not] delete/edit my question, so distorting its sense, after [an] answer to [my] question [has been accepted]?
You can ask. In my view, the question desperately needs editing. There is a good reason why the designers of english.stackexchange designed it to allow people to edit other's questions.
I hope the above helps and does not cause offence, none is intended. If offence is caused, I regret it and apologise for any deficiency in my writing, or carelessness on my part, that caused it.