I am not sure how long this will work because I am sure Automattic will kill the classic editor but at the moment there is still one way to use the old editor. Previously, many people used the redirect script and just clicked on the white “Write” button at the top bar in the dashboard and it worked like a charm. This stopped working now.
Attention: Stop reading! The workaround below is outdated. But for now, there are new workarounds! Since WordPress is constantly making changes, I might already have written new workarounds. So, I created a page where you find the most recent WordPress workarounds. So, check if there are any updates or new posts!
Here might be one final solution and again, I have no clue for how long this will work or if it works for everyone (it seems they shift people to the new editor in phases). Anyway, try it! Open the classic dashboard of your site (yoursite/wp-admin). Then look to the left where the menu is. Like in the image below, you click on “Posts” and then on “Add New”. This will still open the classic editor for now. Once they phased it out, we either need to look for alternatives (self-hosting with a classic fork) or attempt to get used to the miserable Gutenberg editor, which is an impossible mission because it’s that bad.
Diary of Dennis 
I’m actually curious why people are not using this one more often:
Click on Write or New Post > On the writing page, click on the three dots on the top-right corner > Find the Classic Editor option at the bottom of the menu.
Let’s just hope that option doesn’t go away.
It’s already away. If it’s still there in your case, it’s because your user account isn’t switched entirely to Gutenberg yet. They move users in phases as it seems. The option you mean is definitely not there in my case, thus, I am one of the users where it’s already disabled. And in that case, what I wrote in my post is the very last way to get to the real classic editor. 😦