Indeed, I get + 2% to Raw Production for Slave Trade Route on the planet initiating the trade route (Drengi).
But why is the Trade Route Value reduced from 29 to 23?
In the tooltip, the Trade Planet Incomes increased from +2.30 to +3.83, and the Route Length is reduced from +6.90 to +2.76, why?

It does not make sense, the Route Length has not changed.

The trait is supposed to give a bonus not a "penalty".

Per tile bonus should go up, not down:
Basic Trade Route bonus < Luxury Trade Route bonus < Slave Trade Route bonus
Or be added, if the player gets several ideological traits:
Basic Trade Route bonus + Luxury Trade Route bonus + Slave Trade Route bonus

Or I missed something?

I've never been very clear on the slave trade ideology bonus.  Luxury routes make complete sense to me, but not so much the slave trade.  I don't mind them not stacking, but I'd love a breakdown of what actually happens (mathematically) with a slave route.