improve error message when specifying a previously-deprecated frequency alias

Author: jgao8

pandas/_libs/tslibs/offsets.pyx

@@ -5275,6 +5275,16 @@ def _get_offset(name: str) -> BaseOffset:
 
     return _offset_map[name]
 
+deprecated_to_valid_alias = {
+    "H": "h",
+    "BH": "bh",
+    "CBH": "cbh",
+    "T": "min",
+    "S": "s",
+    "L": "ms",
+    "U": "us",
+    "N": "ns",
+}
 
 cpdef to_offset(freq, bint is_period=False):
     """
@@ -5389,6 +5399,8 @@ cpdef to_offset(freq, bint is_period=False):
                         # If n==0, then stride_sign is already incorporated
                         #  into the offset
                         offset *= stride_sign
+                elif name in deprecated_to_valid_alias:
+                    raise ValueError(f"Did you mean '{deprecated_to_valid_alias[name]}'")
                 else:
                     stride = int(stride)
                     offset = _get_offset(name)

pandas/tests/tseries/offsets/test_offsets.py

@@ -1156,6 +1156,9 @@ def test_offset_multiplication(
 
     tm.assert_series_equal(resultarray, expectedarray)
 
+def test_offset_deprecated_error():
+    with pytest.raises(match=r"Did you mean 'h'?"):
+        date_range("2012-01-01", periods=3, freq='H')
 
 def test_dateoffset_operations_on_dataframes(performance_warning):
     # GH 47953