property scope is a new constraint type specifying where a property may be used: in the main snak, in qualifiers, and/or in references. There has been some controversy about the way this constraint is modeled, but from the project talk page there now seems to be general agreement that the constraint parameter for this is the new property scope property.
Should be fairly easy to implement: add support for the parameter to ConstraintParameterParser (returning an array of Context::TYPE_* constants), and then in the new checker simply look at the context’s type.
In the interest of a healthy relationship with the community, I think we should implement this as soon as possible :) however, there will be no deployments on the week of June 18th (SRE offsite), so the next regular deployment where this could happen will be on the evening of June 27th (assuming no train problems). I’ll try to implement this on June 25th and 26th, and hopefully get it merged before the branch cut.