Thanks to Lua scripting capabilities of Redis, we should be able to implement the tag invalidation logic in Redis itself:

On data read, Redis would return the item if its tag versions are up to date. If not Redis would prune the item+its tags. The write + invalidation logic should remain basically the same. We could also implement PruneableInterface on this adapter.

I think this would make Redis the most efficient backend for tagged items.
A similar strategy could also work for a PdoTagAwareAdapter btw.

See https://redis.io/commands/eval