[Answered ]-How to hide certain json data from passing through if it doesn't meet a condition in Django rest framework?

set reply_id field as write_only

class CommentSerializer(serializers.ModelSerializer):
    content = serializers.CharField()
    reply_id = serializers.PrimaryKeyRelatedField(
        source='reply',
        queryset=Comment.objects.all(),
        required=False,
        write_only=True
    )
    user = serializers.ReadOnlyField(source="user.username")
    replies = CommentCreateSerializer(many=True, read_only=True)

    class Meta:
        model = Comment
        fields = ['id', 'user', 'content', 'replies', 'reply_id', 'timestamp']

Serializers validation will work fine in this case.
If you need to add a condition to a specific field then create a method validate_fieldname and if you want to add condition over a specific object then directly create a method as validate.

You can read the documentation for more information – https://www.django-rest-framework.org/api-guide/serializers/#field-level-validation

In your case you need to filter as @Chandan said before. So really you need to refactor your view with the reply_id filter

    permission_classes = (IsAuthenticated,) 
    # pagination_class = PageNumberPagination

    def get(self, *args, **kwargs):
        post                = get_object_or_404(Post, slug=self.kwargs['slug'])
        comments            = Comment.objects.filter(post=post,reply_id__isnull=True)
        paginator           = CustomPaginator()
        response            = paginator.generate_response(comments, CommentSerializer, self.request)
        return response