emacs-lisp ?si= strip script.

si-strip.el

(require 'url)
(require 'cl-lib)
(require 'seq)
;;^~~~~~~~~~~~~~^ comment if inadequate.

(let* ((url "https://youtu.be/dQw4w9WgXcQ?si=GoFuckYourself&t=420") ;; example url
       (url-obj (url-generic-parse-url url))
       (url-ofs (copy-sequence (url-filename url-obj))) 
       ;; url-old-filename-string, used after mutate
       (si-idx (string-match "?si=.*" (url-filename url-obj)))
       (t-idx  (string-match "&t=.*"  (url-filename url-obj))))
  
  (when si-idx
    (setf (url-filename url-obj) 
      (seq-take (url-filename url-obj) si-idx)))
  
  (when t-idx ;; keep and append if present
    (setf (url-filename url-obj)
      (cl-concatenate 'string
        (url-filename url-obj) 
        (seq-drop url-ofs t-idx))))
  ;; serialize.
  (url-recreate-url url-obj))