Some add-ons might create additional Slick tables that store e.g. AtlassianHost.clientKey as a foreign key to to a Atlassian host in their data model.
To write a Slick foreignKey constraint you need access to the referenced Schema (i.e. AtlassianHostTable), see http://slick.lightbend.com/doc/3.2.0/schemas.html#constraints
Therefore I think it's better to make AtlassianHostTable public, so it can be referenced in the constraint.