I've been trying them against every monster I can find, it's both not preventing anything I can find from entering it's border or preventing anything from destroying improvements in that border. The xml still lacks any gamemodifier for them. Placeholders are fine in beta, but it's not beta anymore, these should have been removed by now, or made to work.

    <ImprovementType InternalName="Outpost_Warden">
        <DisplayName>Warden</DisplayName>
        <Description>Most monsters won't enter the outpost's borders.</Description>
        <HideInBuildList>1</HideInBuildList>
        <IsOutpostUpgrade>1</IsOutpostUpgrade>
        <Prereq>
            <Type>Tech</Type>
            <Attribute>Recruiting</Attribute>
        </Prereq>
        <LaborToBuild>166</LaborToBuild>
        <ArtDef>Art_Outpost_Warden</ArtDef>
    </ImprovementType>