I decide to trust users and not assume text by default

src/monty/io.py

@@ -74,11 +74,10 @@ def zopen(
         warnings.warn(
             "We strongly discourage using implicit binary/text `mode`, "
             f"and this would not be allowed after {_deadline}. "
-            "I.e. you should pass t/b in `mode`, we would assume text mode for now",
+            "I.e. you should pass t/b in `mode`.",
             FutureWarning,
             stacklevel=2,
         )
-        mode += "t"  # assume text mode if not specified
 
     # Warn against default `encoding` in text mode
     if "t" in mode and kwargs.get("encoding", None) is None:

tests/test_io.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import os
+import warnings
 from pathlib import Path
 from unittest.mock import patch
 
@@ -237,13 +238,19 @@ def test_warnings(self, extension):
                 f.write(content)
 
             # Implicit text/binary `mode` warning
+            warnings.filterwarnings(
+                "ignore", category=EncodingWarning, message="argument not specified"
+            )
             with (
                 pytest.warns(
                     FutureWarning, match="discourage using implicit binary/text"
                 ),
-                zopen(filename, "r", encoding="utf-8") as f,
+                zopen(filename, "r") as f,
             ):
-                assert f.readline() == content
+                if extension == ".txt":
+                    assert f.readline() == content
+                else:
+                    assert f.readline().decode("utf-8") == content
 
             # Implicit `mode` warning
             with (