Classic editor

You can tell which editor a post uses from its markup. Classic content is plain HTML with paragraph breaks added at display time by wpautop. Block content contains comment delimiters like <!-- wp:paragraph -->. Settings > Writing shows a Default editor for all users option once the Classic Editor plugin is active.

On a post list, the plugin adds an extra Classic Editor edit link and a Switch to Block Editor option if you allow users to choose. The plugin's active-install count on WordPress.org runs into the millions, which gives a rough scale of how many sites still depend on it.

A university department site has 3,200 pages built over a decade with shortcodes and pasted HTML tables. Opening a typical page in the block editor turns each table into a Classic block and each shortcode into a Shortcode block. Nothing breaks, but the editor is slow.

The IT team keeps the Classic Editor plugin active, sets it as the default, and allows the block editor only for the new news section.

The classic editor stores one body of HTML, while Gutenberg stores structured blocks with metadata in comments. Moving a post from one to the other is lossless in most cases, but classic markup converts to a Classic block rather than to separate paragraph and heading blocks until someone converts it. Gutenberg is core; the classic editor is a plugin you must keep updated.

Installing the plugin and forgetting to set the default, so editors see both. Assuming plugin support means a custom meta box will behave the same in the block editor. Pasting from Word into the Code tab. Removing the plugin and finding every post now opens as a single Classic block. Expecting page builders to depend on it when they bring their own editor.

Decide per content type. If the site is mostly legacy pages, keep the plugin and stop worrying. If you plan to move, convert one section to blocks on staging, check the front end with and without your theme's editor styles, and set a date. Do not leave both editors available to everyone.