It looks like Discover Weekly is not longer accessible.
Trying to get it by ID returns a 404 and searching or getting playlists returns a list with a null value where I expect the playlist to be.
I don't remember that it was included by default. When they decided to add it, they offered it to existing customers and made it available for new customers if they paid more than the average price.
They made it clear:
"For all ~180,000 customers, this is complimentary."
and
"For new customers, we're trying an experiment: to unlock the Humble Bundle #1 inside of your bundle, just choose a price that is above average"