Block widgets, and the filter that opts out

5.8 replaced the widget screen with a block editor, and six custom widgets became one HTML block each with the rendered markup pasted in.

// opt out entirely — both screens, and the customiser
add_filter( 'use_widgets_block_editor', '__return_false' );

// or per screen
remove_theme_support( 'widgets-block-editor' );

// this is a deferral with no expiry attached to it.
// write the date you intend to revisit it in a comment.

The opt-out is legitimate and is a decision to make deliberately rather than by reflex, because the deadline is set by somebody else — the filter will not exist forever. The migration work is real: a widget is a PHP class with a form and an update method, and a block is a JavaScript component with attributes, and there is no automatic conversion. Testing an upgrade on a copy of a real site, widget by widget, is the only way to find out how much work it is.