@EzekielRaiden brought up the X-card as an example of a mechanic that actively helps a goal, as opposed to passive neutrality. I agree that is accurate, but in the context of sandbox campaigns, the X-card, and most RPG mechanics, are simply tools that are neutral in their effect on whether a system is suitable for a sandbox campaign.
That whether a safety tool like the X-card makes sandbox play easier or harder depends entirely on how the group chooses to implement it, not on anything inherent to the tool itself.
That's the point I was trying to make earlier: the X-card (along with another specific mechanic) is usually irrelevant to the structural question of whether a system supports sandbox play. What makes a campaign a sandbox campaign is
how those mechanics are used in refereeing it.
You make a critical comment in your reply to
@pemerton
The bulk of my work promoting sandbox campaigns for the past decades has been about turning vague suggestions and ideas into concrete techniques and advice to help folks run sandbox campaigns regardless of the system they are using.
I studiously avoid framing my essays and how-tos as rules or as a system.
Instead they form a body of techniques and a method of organization that are meant to be applied to the unique circumstances of a campaign to help the referee and the group manage a sandbox campaign. Not a one-size-fits-all ironclad system.
As a general note, the hobby and industry would benefit more from folks writing good refereeing advice for all types of campaigns rather than trying to codify style and feel into systems.
I hope this addresses your points.