wp-admin/includes/class-wp-screen.php:461Sets or returns whether the block editor is loading on the current screen.
$setbooloptionalnullbool public function is_block_editor( $set = null ) { if ( null !== $set ) { $this->is_block_editor = (bool) $set; } return $this->is_block_editor; }Introduced in 5.0.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-admin/includes/class-wp-screen.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.