Include from your theme file. Use [footnote]This is the content of my footnote.[/footnote] to add a footnote.

footnotes-for-wordpress.php

<?php
/**
 * Footnotes
 */
function footnote($atts, $content = '') {
  global $footnote_number, $footnote_contents;

  $footnote_number ??= 0;
  $footnote_contents ??= [];
  $footnote_number++;
  $post_id = $GLOBALS['post']->ID;

  $footnote_name = "footnote-$post_id-$footnote_number";
  $footnote_ref = "footnote-ref-$post_id-$footnote_number";
  $inline_content = esc_attr(wp_strip_all_tags($content));

  $footnote_contents[] = [
    'id'      => $footnote_number,
    'name'    => $footnote_name,
    'ref'     => $footnote_ref,
    'content' => $content,
  ];
  return "<sup><a name=\"$footnote_ref\" href=\"#$footnote_name\" title=\"$inline_content\" class=\"footnote-ref\">$footnote_number</a></sup>";
}

function footnotes_reset_count($content){
  global $footnote_number;

  $footnote_number = 0;
  $footnote_contents = [];

  return $content;
}

function footnotes_display_after_content($content){
  global $footnote_contents;

  if( ! $footnote_contents ) return $content;
  if( empty(  $footnote_contents ) ) return $content;

  $content .= "<div class=\"footnotes\" style=\"font-size: 80%; margin-top: 0.5em; padding-top: 0.5em; border-top: 1px solid #666;\"><h2 class=\"sr-only\">Footnotes</h2>";
  foreach($footnote_contents as $footnote) {
    $content .= "<p class=\"footnote\" data-footnote-name=\"${footnote['name']}\"><sup><a name=\"${footnote['name']}\" href=\"#${footnote['ref']}\">${footnote['id']}</a></sup> ${footnote['content']}</p>";
  }
  $content .= "</div>";

  $footnote_number = 0;
  $footnote_contents = [];

  return $content;
}

add_shortcode('footnote', 'footnote');
add_filter('the_content', 'footnotes_reset_count', 10);
add_filter('the_content', 'footnotes_display_after_content', 11);

function allow_shortcodes_in_captions($out, $pairs, $atts) {
  $out['caption'] = do_shortcode($out['caption']);
  return $out;
}
add_filter('shortcode_atts_caption', 'allow_shortcodes_in_captions', 10, 3);