Problem/Motivation
Similar as we want to expose UPDATE functionality for config entities, (see #2869792: Add constraints to all config entity types / #2300677: [PP-1] POST/PATCH/DELETE config entities via REST for config entity types that support validation) we want to also add support for simple configuration, see #2911319: Provide a single command to install & run Drupal / #2949888: Enhance config schema for richer default experiences).
In order to safely expose configuration entities we need to add constrains to simple configurations, like the system.site configuration.
Proposed resolution
Let's add an issue for simple configuration so we can add the constrains as needed.