per feature flag:

definition user {}

definition platform {
  relation feature_somecoolfeature: user | user:*
}

and then your check could be

platform:platformname#feature_somecoolfeature@user:currentuser

. The benefit of this design is you can grant specific features to specific users, or to

user:*

to enable it for everyone. you also can in theory pull the feature flag permission into other permissions, so you could do something like enable new permissions based on a feature being turned on (or turned off) for a specific user