As Frank indicates in his comment on your post, the return statement that is producing the warning is this one:

if (post.sanitized) return;

The warning can be silenced by returning a dummy value (e.g. null, false, 0). The value is not used.

Earlier versions of Cloud Functions did not complain when a function exited using a return statement with no value. That explains why you see return; in the video you linked and in the documentation. The comment on the question by Firebaser Frank van Pufeelen, explains why the change was made.

The simplest way to eliminate the warning is to add a return value, as Frank suggested:

if (post.sanitized) return 0;

Another option is to change the trigger from onWrite() to onCreate(). Then the function will not be invoked when the post is sanitized and the check that produces the warning is not needed:

exports.sanitizePost = functions.database

    .ref('/test/{pushId}')

    .onCreate(event => {  // <= changed from onWrite()

        const post = event.data.val();

        //if (post.sanitized) return; // <= no longer needed



        console.log('Sanitizing new post', event.params.pushId);

        console.log(post);

        //post.sanitized = true; // <= not needed when trigger is onCreate()

        post.title = sanitize(post.title);

        post.body = sanitize(post.body);

        return event.data.ref.set(post);

    });