Classic Editor had been the default editor of WordPress since its initial release until it was replaced by Gutenberg in WordPress version 5.0 which released in 2018. Although Gutenberg offers a more modern content editing experience, not all users really welcome it. Some users prefer to stick to Classic Editor. If you are the member of latter user group, you can disable Gutenberg to revert to the Classic Editor. How to do so?
Although Classic Editor has been replaced by Gutenberg, it is not really thrown away. Classic Editor is still around on the WordPress system. You can get it back by disabling Gutenberg.
Reverting to Classic Editor without Plugin
The WordPress developer team offers a plugin in case you want to stick to Classic Editor. You can simply install and activate the plugin to switch back to Classic Editor. Alternatively, you can disable Gutenberg by adding a new function to the functions.php file of your theme.
First, login to your WordPress dashboard and go to Appearance -> Theme File Editor. Select the active theme, then click the functions.php file of the associated theme to edit it.
Add the following code snippet to the file. You can place it at the bottom of the file content. Click the Update File button to apply the change.
add_filter('use_block_editor_for_post', '__return_false', 10);
Once done, try to edit a new content (page or post). You should get Classic Editor back on your website.