I don't speak English well. So I have just tried to start the discuss about the problem. When I faced to issue I usually use "Give Feedback" menu. But I have not received answers never.
When we upgraded from Jira 7.5.0 to 8.1.0, I knew that there were going to be some users who were going to hate the new interface. These are also the same users that hated the "new" interface that last time there was a major UI update, and these are the same people that are now requesting to go back to that which they hated previously.
What I discovered is that they don't hate the new UI as much as they had grown accustomed to the way things were and how they interacted with the product on a day to day basis.
I agre with @Mirek are there any specific things that you dislike? Saying the whole thing is terrible makes it hard to know what can/needs to be improved to make it easier for you to use.
For example, some hotkeys was broken. Hotkeys "." and "w" doesn't works right now.
Moreover, some of old function doesn't works right in old interface. For example, if someone are inserting pictures using new interface those pictures doesn't fit to screen on old interface.