Deprecate the implicit_value option registration kwarg. (Cherry-pick of #20757) (#20758)

It was never used in the Pants repo, and was only implemented back in
2015, as part of removing our use of argparse, for feature parity.

It is undocumented, and not supported by the newer BoolOption
registration mechanism. So the only place this could be used is
in old-style option registration in a third-party plugin, which seems
pretty unlikely.

However, for good practice, we give a short deprecation period, just in
case.

Co-authored-by: Benjy Weinberger <[email protected]>

Author: WorkerPants

src/python/pants/option/parser.py

@@ -17,7 +17,7 @@
 import yaml
 
 from pants.base.build_environment import get_buildroot
-from pants.base.deprecated import validate_deprecation_semver, warn_or_error
+from pants.base.deprecated import deprecated_conditional, validate_deprecation_semver, warn_or_error
 from pants.option.config import DEFAULT_SECTION, Config
 from pants.option.custom_types import (
     DictValueComponent,
@@ -415,6 +415,13 @@ def error(
                 error(OptionNameDoubleDash, arg_name=arg)
 
         # Validate kwargs.
+        deprecated_conditional(
+            lambda: "implicit_value" in kwargs,
+            removal_version="2.21.0a0",
+            entity="The 'implicit_value' option registration keyword",
+            hint="Command line flags specifying this option must provide its value explicitly, "
+            "except for boolean flags, where --foo is the same as --foo=true.",
+        )
         if "implicit_value" in kwargs and kwargs["implicit_value"] is None:
             error(ImplicitValIsNone)
         type_arg = kwargs.get("type", str)