I guess my English texts usually are dead

I'm not sure what you mean by "dead" here.

A few weeks ago, I was writing an email to one ya YoutTuber.

"one" sounds like you're emphasising it was only one.

I was amazed by his work, and I[I] felt some kind of necessity to write him a few words of appreciation (also I wanted to share with the idea of my notetaking system, but that is another story).

You could not have the second "I".

I think I tend to hear "felt some kind of need" - "necessity" feels slightly formal, but it's not wrong.

I think "I also wanted" sounds slightly more natural.

or "share the idea of my notetaking system with him"

"that is another story" sounds very natural here, good job. Another thing you could say here is "but that's getting off-topic".

Back in my days I was writingrote letters, but that was as a generic exercise in the school at/formulaic/prescriptive exercise in English classes.

I know what you mean by "generic", I think "formulaic" or "prescriptive" might fit better.

Little did I know that a letter in English willould be the subject of my concerns in the future.

Or "would be a future subject of concern".

I sent it,/the email and after Iwards [I] copied my written ing/text and pasted it in ChatGPT.

I'd say "the email" here rather than "it", as it's been a little time since you mentioned it.

"afterwards" sounds better to me, but I can't tell you why. I don't think "after" is wrong though.

"written text" sounds clunky

or "afterwards copied and pasted my writing in ChatGPT".

Just inas an academic interest.

SIn shortly: I was really disappointed.

Hey everybody .¶
Thanks to everyone who spends time on [reading and] proofreading.

You could skip "reading" as it's implied in "proofreading".

I guess my English texts are usually are dead

(1) "usually are" is okay, but "are usually" sounds more natural.
(2) Not too sure what you meant by "dead" here.

A few weeks ago I was writing an email to one ya YoutTuber.

(1) "a" is more natural than "one" here, since the emphasis is not on the number of YouTubers you wrote to, but rather the fact that you wrote to a YouTuber.
(2) "YouTube" is a proper noun, so "YouTuber" should follow its capitalisation.

I was amazed by his work, and I felt some kind of necessity to write him a few words of appreciation. (aAlso, I wanted to share with him the idea of my notetaking system, but that is another story).)

The phrase is parentheses is a whole sentence in of itself, and should be written as such. (Yes, you can put entire sentences in parentheses.)

Back in my days I was writing letters, but that was a generic exercise in the school atduring English classes (in school).

"In school" may be omitted, since "English classes" already implies that it's in school.

Little did I know that a letter in English willould be the subject of my concerns in the future.

It should be "would", and not "will", since "little did I know" is in the past tense.

I sent it, and after that I copied my written texting and pasted it into ChatGPT.

(1) "Written text" is okay, but "writing" is more concise and probably more natural.
(2) "in ChatGPT" is probably okay, but allow me to suggest "into ChatGPT". "In" might only suggest that you pasted the text, but "into" suggests that you pasted it and then fed it into the language model for an output.

I asked it to "optimize" the text or rephrase it.

Just in an academic interestout of curiosity.

I'm not 100% sure what you meant by your original sentence, but I took my best guess at it.

SIn shortly: I was really disappointed.

"Shortly", while possible, will be interpreted in this context by most readers to mean "in a short time; soon." Using "in short" clears up any ambiguity, since "in short" has only one definition.

And here I am .

Hey everybody .¶
Thanks to everyone who spends time on reading and proofreading.

(1) The "on" is okay, but may be omitted. I think it sounds more natural that way.
(2) The "reading" is redundant, since "proofreading" already necessitates reading.