Anybody know why logical replication isn't supported in Aurora Postgres, or if it's on the roadmap?
I don't know, but my guess is that they changed too much of the storage layer to keep it working. Logical decoding gets its data from postgres' write ahead log - which aurora supposedly largely replaced.
It's currently PostgreSQL 9.6.3 compatible. I imagine they'll aim for 10 at some point (logical replication isn't built in in 9.6.3)
Note that RDS supports, before 10, streaming out logical changes. Support for that was added to postgres 9.4. I stand by my statement that it'll be problematic for Aurora to support this.