[youtube] Hardcode codec metadata for av01 video only formats (closes #21381)

youtube_dl/extractor/youtube.py

@@ -500,6 +500,12 @@ class YoutubeIE(YoutubeBaseInfoExtractor):
 
         # RTMP (unnamed)
         '_rtmp': {'protocol': 'rtmp'},
+
+        # av01 video only formats sometimes served with "unknown" codecs
+        '394': {'acodec': 'none', 'vcodec': 'av01.0.05M.08'},
+        '395': {'acodec': 'none', 'vcodec': 'av01.0.05M.08'},
+        '396': {'acodec': 'none', 'vcodec': 'av01.0.05M.08'},
+        '397': {'acodec': 'none', 'vcodec': 'av01.0.05M.08'},
     }
     _SUBTITLE_FORMATS = ('srv1', 'srv2', 'srv3', 'ttml', 'vtt')