Deprecate returning False from validators

CHANGELOG.rst

@@ -15,6 +15,8 @@ Bug fixes:
 
 Deprecations:
 
+- Custom validators should raise a `ValidationError <marshmallow.exceptions.ValidationError>` for invalid values. 
+  Returning `False`` is no longer supported .
 - Deprecate ``context`` parameter of `Schema <marshmallow.schema.Schema>` (:issue:`1826`).
   Use `contextVars.ContextVar` to pass context data instead.
 - `Field <marshmallow.fields.Field>`, `Mapping <marshmallow.fields.Mapping>`,

pyproject.toml

@@ -90,3 +90,6 @@ no_implicit_optional = true
 [tool.pytest.ini_options]
 norecursedirs = ".git .ropeproject .tox docs env venv tests/mypy_test_cases"
 addopts = "-v --tb=short"
+filterwarnings = [
+  "ignore:Returning `False` from a validator:marshmallow.warnings.ChangedInMarshmallow4Warning",
+]

src/marshmallow/validate.py

@@ -4,12 +4,14 @@
 
 import re
 import typing
+import warnings
 from abc import ABC, abstractmethod
 from itertools import zip_longest
 from operator import attrgetter
 
 from marshmallow import types
 from marshmallow.exceptions import ValidationError
+from marshmallow.warnings import ChangedInMarshmallow4Warning
 
 _T = typing.TypeVar("_T")
 
@@ -77,6 +79,12 @@ def __call__(self, value: typing.Any) -> typing.Any:
             try:
                 r = validator(value)
                 if not isinstance(validator, Validator) and r is False:
+                    warnings.warn(
+                        "Returning `False` from a validator is deprecated. "
+                        "Raise a `ValidationError` instead.",
+                        ChangedInMarshmallow4Warning,
+                        stacklevel=2,
+                    )
                     raise ValidationError(self.error)
             except ValidationError as err:
                 kwargs.update(err.kwargs)