Generally Gutenberg block editor sucks. It doesn’t bring any prominent advantages at this day and creates multiple confusions. Needless to say one can NOT edit URL slug unless a post/page is published. The video embedding is confusing etc.
To both upgrade to the latest version of WordPress and have regular editor (vs Gutenberg) as we accustomed to, we need to edit functions.php by inserting the code below:
There are other options like installing special plugins, but that’s too much just for disabling something.
If you need more info and other solutions, read this article https://kinsta.com/blog/disable-gutenberg-wordpress-editor/