[cleanup] Deprecate more compat functions (#11439)

Authored by: seproDev

yt_dlp/utils/_legacy.py

@@ -313,3 +313,30 @@ def make_HTTPS_handler(params, **kwargs):
 
 def process_communicate_or_kill(p, *args, **kwargs):
     return Popen.communicate_or_kill(p, *args, **kwargs)
+
+
+def encodeFilename(s, for_subprocess=False):
+    assert isinstance(s, str)
+    return s
+
+
+def decodeFilename(b, for_subprocess=False):
+    return b
+
+
+def decodeArgument(b):
+    return b
+
+
+def decodeOption(optval):
+    if optval is None:
+        return optval
+    if isinstance(optval, bytes):
+        optval = optval.decode(preferredencoding())
+
+    assert isinstance(optval, str)
+    return optval
+
+
+def error_to_compat_str(err):
+    return str(err)