An example of a plugin that chops content the pre Gutenberg way

content_chopper.php

<?php
    /*
    Plugin Name: Content Chopper
    Plugin URI: https://github.com/rachelslurs
    Description: A simple content chopper
    Version: 1.0
    Author: Rachel Cantor
    Author URI: https://github.com/rachelslurs
    License: GPL2
    */
    
    class ContentChopper {
        private $plugin_path;
        private $plugin_url;
        function __construct() {	
            $this->plugin_path = plugin_dir_path( __FILE__ );
            $this->plugin_url = plugin_dir_url( __FILE__ );
            require_once( plugin_dir_path( __FILE__ ) . '/htmlawed.php' );
            add_action( 'init', array(&$this, 'init') );
        }
        
        public static function init() {
            //$more_is_being_used = strpos($content, 'class="more-link">') !== false;
            if (is_singular()) {
                wp_die('hello');
                add_filter( 'the_content', array(&$this, 'block_extra_content_from_displaying' ) );
            }
        }
        
        function block_extra_content_from_displaying( $content ) {
            global $post;
            $chunks = self::chop_it($post);

            // We need to temporarily remove our filter so we can apply the other filters without an infinite loop
            $content_needs_chopping = false;

            if ( has_filter( 'the_content', array(&$this, 'block_extra_content_from_displaying' ) ) ) {
                remove_filter( 'the_content', array(&$this, 'block_extra_content_from_displaying' ) );
                $content_needs_chopping = true;
            }

            // Run it through the Wordpress gauntlet
            $snippet = apply_filters( 'the_content', $chunks['snippet'] );

            // add our filter back in
            if ( $content_needs_chopping ) {
                add_filter( 'the_content', array(&$this, 'block_extra_content_from_displaying' ) );
            }

            $snippet = clean_snippet($snippet);


		    return "<pico>$snippet</pico>";
        }

        function chop_it($post) {
            $length_type = 'words'; // words or characters
            $finish = 'sentence'; // exact, word, or sentence
            $post_body = wpautop($post->post_content);
            $p_tag = strpos( $post_body, '</p>' ); // find the string position of the first instance of '</p>', else false
            $length = 40; // use this if $p_tag is false

            $extended_content = get_extended( $post_body );
            $more_is_being_used = $extended_content['extended'] ? true : false;

            $unfiltered_content = trim($post_body);

            // If "read more" is on, set the snippet to the first extended content chunk; if not, we cut it ourselves
            if ( $more_is_being_used) {
                    $snippet = $extended_content['main'];
                } else {
                // if the p tag is found, set snippet to be the first paragraph, else fallback to the old way of doing excerpts (40 words, ends on a sentence)
                    $snippet = !!$p_tag ? self::get_first_paragraph($unfiltered_content, $p_tag) : self::text_excerpt( $unfiltered_content, $length, $length_type, $finish );
                }

            $len = strlen($snippet);
            $remainder = trim( substr( $unfiltered_content, $len) );
            // $remainder = reverse_balance_tags($remainder);
                wp_die($snippet);
            return htmLawed($snippet);
        }

        function get_first_paragraph($text, $position_of_closing_p_tag) {
            return substr($text, 0, $position_of_closing_p_tag) . '</p>';
        }

        function text_excerpt( $text, $length, $length_type, $finish ) {
            $tokens = array();
            $result = '';
            $w = 0;

            // Divide the string into tokens; HTML tags, or words, followed by any whitespace
            // (<[^>]+>|[^<>\s]+\s*)
            preg_match_all( '/(<[^>]+>|[^<>\s]+)\s*/u', $text, $tokens );
            foreach ( $tokens[0] as $t ) { // Parse each token

                if ( $w >= $length && 'sentence' != $finish ) { // Limit reached
                break;
                }

                if ( $t[0] != '<' ) { // Token is not a tag
                if ( $w >= $length && 'sentence' == $finish && preg_match( '/[\?\.\!]\s*$/uS', $t ) == 1 ) { // Limit reached, continue until ? . or ! occur at the end
                    $result .= trim( $t );
                    break;
                }

                if ( 'words' == $length_type ) { // Count words
                    $w++;
                } else { // Count/trim characters
                    $chars = trim( $t ); // Remove surrounding space
                    $c = strlen( $chars );
                    if ( $c + $w > $length && 'sentence' != $finish ) { // Token is too long
                    $c = ( 'word' == $finish ) ? $c : $length - $w; // Keep token to finish word
                    $t = substr( $t, 0, $c );
                    }
                    $w += $c;
                }
                }
                // Append what's left of the token
                $result .= $t;
            }

            return $result;
        }
    }

    new ContentChopper();
?>