re-add deprecated Config.from_json method

CHANGES.rst

@@ -23,6 +23,8 @@ Unreleased
 -   Roll back a change to the order that URL matching was done. The
     URL is again matched after the session is loaded, so the session is
     available in custom URL converters. :issue:`4053`
+-   Re-add deprecated ``Config.from_json``, which was accidentally
+    removed early. :issue:`4078`
 
 
 Version 2.0.0

src/flask/config.py

@@ -202,6 +202,31 @@ class Config(dict):
 
         return self.from_mapping(obj)
 
+    def from_json(self, filename: str, silent: bool = False) -> bool:
+        """Update the values in the config from a JSON file. The loaded
+        data is passed to the :meth:`from_mapping` method.
+
+        :param filename: The path to the JSON file. This can be an
+            absolute path or relative to the config root path.
+        :param silent: Ignore the file if it doesn't exist.
+
+        .. deprecated:: 2.0.0
+            Will be removed in Flask 2.1. Use :meth:`from_file` instead.
+            This was removed early in 2.0.0, was added back in 2.0.1.
+
+        .. versionadded:: 0.11
+        """
+        import warnings
+        from . import json
+
+        warnings.warn(
+            "'from_json' is deprecated and will be removed in Flask"
+            " 2.1. Use 'from_file(path, json.load)' instead.",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        return self.from_file(filename, json.load, silent=silent)
+
     def from_mapping(
         self, mapping: t.Optional[t.Mapping[str, t.Any]] = None, **kwargs: t.Any
     ) -> bool: