How would one limit the Manual Excerpt Length in WP?

Question

One can control the WP default (Automatic) excerpt length of a WP post using the using the following snippet within functions.php;

From the WP Codex

// . Post excerpt adjustment (Auto)
// . ==============================
function wpdocs_custom_excerpt_length( $length ) {
    return 20;
}
add_filter( 'excerpt_length', 'wpdocs_custom_excerpt_length', 999 );

My question is how do you limit the manual one?

You know, the exerpt specifically added by the user themselves?

(*) There is an 8 year old question here, that does provide some context but given the current year and progress WP has made I want to post the question again and receive some clarity on the subject.

Added Context: (Edited: 12 March 2019)

It's not that the original answer to the question posted earlier doesn't work, all be it seems, really clunky. I'm looking for a more simple & robust answer using exerpt_length filter. Rather than using something like the following to trim the text; (If Possible)

function excerpt($limit) {
    return wp_trim_words(get_the_excerpt(), $limit);
}

Answer 1

We have by default in core, the following filtering:

add_filter( 'get_the_excerpt', 'wp_trim_excerpt' );

but within wp_trim_excerpt() the trimming is only applied on the post's content, when there's no manual excerpt set.

Here's an untested suggestion for a custom filtering:

add_filter( 'get_the_excerpt', function( $excerpt, $post ) {
    if ( has_excerpt( $post ) ) {
        $excerpt_length = apply_filters( 'excerpt_length', 55 );
        $excerpt_more   = apply_filters( 'excerpt_more', ' ' . '[…]' );
        $excerpt        = wp_trim_words( $excerpt, $excerpt_length, $excerpt_more );
    }
    return $excerpt;
}, 10, 2 );

to apply the similar trimming on manual excerpts.

Hope you can adjust this further to your needs.