diff --git a/stix2/base.py b/stix2/base.py
index 375030d..d905fb0 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -16,9 +16,7 @@ from .exceptions import (
 )
 from .markings import _MarkingsMixin
 from .markings.utils import validate
-from .serialization import (
-    STIXJSONEncoder, STIXJSONIncludeOptionalDefaultsEncoder, serialize,
-)
+from .serialization import STIXJSONEncoder, serialize
 from .utils import NOW, PREFIX_21_REGEX, get_timestamp
 from .versioning import new_version as _new_version
 from .versioning import revoke as _revoke
@@ -28,10 +26,6 @@ try:
 except ImportError:
     from collections import Mapping
 
-# TODO: Remove STIXJSONEncoder, STIXJSONIncludeOptionalDefaultsEncoder, serialize from __all__ on next major release.
-# Kept for backwards compatibility.
-__all__ = ['STIXJSONEncoder', 'STIXJSONIncludeOptionalDefaultsEncoder', '_STIXBase', 'serialize']
-
 DEFAULT_ERROR = "{type} must have {property}='{expected}'."
 SCO_DET_ID_NAMESPACE = uuid.UUID("00abedb4-aa42-466c-9c01-fed23315a9b7")
 
diff --git a/stix2/v20/common.py b/stix2/v20/common.py
index d68b4a7..4b7c299 100644
--- a/stix2/v20/common.py
+++ b/stix2/v20/common.py
@@ -74,7 +74,6 @@ class TLPMarking(_STIXBase20):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709287>`__.
     """
 
-    # TODO: don't allow the creation of any other TLPMarkings than the ones below
     _type = 'tlp'
     _properties = OrderedDict([
         ('tlp', StringProperty(required=True)),
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index 12e70f2..83d6fa4 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -91,9 +91,9 @@ class LanguageContent(_STIXBase21):
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond', precision_constraint='min')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond', precision_constraint='min')),
         ('object_ref', ReferenceProperty(valid_types=["SCO", "SDO", "SRO"], spec_version='2.1', required=True)),
-        # TODO: 'object_modified' it MUST be an exact match for the modified time of the STIX Object (SRO or SDO) being referenced.
+        # TODO: 'object_modified' MUST be an exact match for the modified time of the STIX Object being referenced
         ('object_modified', TimestampProperty(precision='millisecond')),
-        # TODO: 'contents' https://docs.google.com/document/d/1ShNq4c3e1CkfANmD9O--mdZ5H0O_GLnjN28a_yrEaco/edit#heading=h.cfz5hcantmvx
+        # TODO: Implement 'contents' property requirements as defined in STIX 2.1 CS02
         ('contents', DictionaryProperty(spec_version='2.1', required=True)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 6fb218c..a9cf2ae 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -468,7 +468,6 @@ class HTTPRequestExt(_Extension):
 
 
 class ICMPExt(_Extension):
-    # TODO: Add link
     """For more detailed information on this object's properties, see
     `the STIX 2.1 specification <https://docs.oasis-open.org/cti/stix/v2.1/cs02/stix-v2.1-cs02.html#_ozypx0lmkebv>`__.
     """