Greasy Fork

Pre 2024 Youtube UI may be returned with next custom rules in ad-blockers:

• AdGuard
  youtube.com#%#//scriptlet('set-constant', 'yt.config_.EXPERIMENT_FLAGS.kevlar_watch_grid', 'false')
• uBO
  youtube.com##+js(set, yt.config_.EXPERIMENT_FLAGS.kevlar_watch_grid, false)

Does it also change the proportions to the old ones?
The script also changes the flags but the proportions needed to be handled independently.

javascript:(function(){
  var player=document.querySelector("#movie_player");
  var video=player.querySelector("video");
  video.style.height=player.offsetHeight+"px";
  video.style.width=player.offsetWidth+"px";
  document.querySelector(".ytp-chrome-bottom").style.width=(player.offsetWidth-24)+"px";
})();

Not sure it's perfect, but it seems like an option...