Logical decoding on a standby node
A patch was submitted for PostgreSQL 10, but didn't get in.
Work continues for PostgreSQL 11.
Clients like pglogical will need modification to support streaming from a standby once the basic functionality is in postgres.
A bit late...
It's essentially just unsupported because nobody had the time to implement it. There's a bunch of complications around reserving the required resources on the primary, and about continuing to correctly work when the standby, or one in a cascade above, is promoted. Nothing impossible, just work.
That kind of question you're probably more likely to get an answer about on the postgresql lists themselves.