Here is a translation of her letter:
Hello, this is Ko Yujin.
I have left my company. I wanted to debut but it didn't turn out the way I wanted.
I had to follow luck and timing and there were many bad things that happened so I stopped because it was so mentally tiring.
I can't say much details about this currently, but I will tell you as soon as I am able to.
I've been living the trainee life for a long time and worked hard but it's been hard day by day ever since I felt like this life was eating away at me. So I gave up on my longtime dream and I'm very sorry to the fans who have waited for my debut, but I think I will live as a non-celebrity in the future.
Also I would like to say one more thing, I didn't try to deceive fans by making it look like I would debut. Due to personal reasons, I couldn't mention the company or debuting at all. To express my thanks to the fans who have been continuously supporting and loving me as myself during this time, I did my best by opening my YouTube and Instagram, doing AfreecaTV live broadcasts, etc. and in the future plan to communicate with the people who can like me not as an idol trainee but as a non-celebrity and show you good contents and things I am good at in order to repay the love I have received thus far. Because I'm going to be a non-celebrity from now on, I won't be mentioning the situation that's currently a problem. There are many things I may receive hurt by as someone who isn't a public figure. Please refrain from malicious rumors or personal attacks that are not factual. In a situation that goes too far, I would have no choice but to take legal action.
Once again, thank you so much to all those who have waited for me for a long time, and please understand that I couldn't immediately talk about my current situation, sorry.
Hopefully she can share more information when she won’t get in trouble :( I’m not a Produce fan but the few clips I saw of her she really caught my attention, it’s sad she had a negative experience that is all too common in this industry.
Sources: News and translation from u/randomneeess on r/kpop, Reddit, @me_ow_yu