Source

File: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

public function delete_item( $request ) {
		// We don't support delete requests in multisite.
		if ( is_multisite() ) {
			return new WP_Error(
				'rest_cannot_delete',
				__( 'The user cannot be deleted.' ),
				array( 'status' => 501 )
			);
		}

		$user = $this->get_user( $request['id'] );

		if ( is_wp_error( $user ) ) {
			return $user;
		}

		$id       = $user->ID;
		$reassign = false === $request['reassign'] ? null : absint( $request['reassign'] );
		$force    = isset( $request['force'] ) ? (bool) $request['force'] : false;

		// We don't support trashing for users.
		if ( ! $force ) {
			return new WP_Error(
				'rest_trash_not_supported',
				/* translators: %s: force=true */
				sprintf( __( "Users do not support trashing. Set '%s' to delete." ), 'force=true' ),
				array( 'status' => 501 )
			);
		}

		if ( ! empty( $reassign ) ) {
			if ( $reassign === $id || ! get_userdata( $reassign ) ) {
				return new WP_Error(
					'rest_user_invalid_reassign',
					__( 'Invalid user ID for reassignment.' ),
					array( 'status' => 400 )
				);
			}
		}

		$request->set_param( 'context', 'edit' );

		$previous = $this->prepare_item_for_response( $user, $request );

		// Include user admin functions to get access to wp_delete_user().
		require_once ABSPATH . 'wp-admin/includes/user.php';

		$result = wp_delete_user( $id, $reassign );

		if ( ! $result ) {
			return new WP_Error(
				'rest_cannot_delete',
				__( 'The user cannot be deleted.' ),
				array( 'status' => 500 )
			);
		}

		$response = new WP_REST_Response();
		$response->set_data(
			array(
				'deleted'  => true,
				'previous' => $previous->get_data(),
			)
		);

		/**
		 * Fires immediately after a user is deleted via the REST API.
		 *
		 * @since 4.7.0
		 *
		 * @param WP_User          $user     The user data.
		 * @param WP_REST_Response $response The response returned from the API.
		 * @param WP_REST_Request  $request  The request sent to the API.
		 */
		do_action( 'rest_delete_user', $user, $response, $request );

		return $response;
	}