Before May 1, 2018, Pages API endpoints relied on App-Scoped User IDs (ASID). Apps created before this date continued receiving ASIDs in endpoint responses, unless an app admin used the App Dashboard to opt-in to receive Page-Scoped User IDs (PSID). Starting with v7.0+, Pages API endpoints will only accept and return PSIDs for all apps, regardless of their creation date. This change will also be applied to all API versions on May 5th, 2021.
The Page-Scoped ID API allows you to map ASIDs to their PSID equivalents. The API is deprecated in v7.0+, but can be used with older versions until May 5th, 2021.