I updated to WordPress 5, but had to use the classic editor plugin. After updating a local copy of my site, I opened an existing post for editing, and was greeted with the HTML content of the post in a text area. I use the markdown feature of the Jetpack plugin, and consider the markdown version of my posts to be the canonical version, even though it’s stored in the post_content_filtered field. So I looked all over for a way to switch to markdown mode, and looked for instructions online, to no avail. The blocks sidebar was empty. I was considering reverting, but decided to try the classic editor plugin I saw mentioned. After enabling, everything was back to normal, with my post content in good old markdown again. Looks and functions exactly like it used to.
For composer users, the classic editor plugin is just:
"wpackagist-plugin/classic-editor": "^1.2"
I’m hoping things will eventually fix themselves and I’ll be able to switch to the new editor: Not because I want blocks or anything like that, but to get rid of a dependency. Maybe my theme needs to be updated for blocks or something. But I’m certainly not switching if my existing posts aren’t in markdown automatically.