Alternator TTL doesn't work when tablets are enabled, because:

1. It uses a per-keyspace replication map.
2. Also this map can change over time while the expiration thread is running. Note that vnodes also had range movements, so the code might be fine - but maybe because the movements are more common now we can have a problem where some data never gets expired because it keeps moving around

See #16203 (comment) for @tgrabiec's comment about this.