However, there are some questions, which should be decided:
Should we keep suport for non-string keys at the yii\caching\CacheInterface?
They are widely used in framework code, while technically contradict PSR-16. Also note that methods like getMultiple() and setMultiple() does not support non-scalar keys even now.
Should be keep add*() methods?
These methods were added to support race-condition-avoid operations provided by some cache storages. However making solution PSR-compatible it is no longer possible to use low-level cache storage methods for that.