Let me clear this all up for you guys. I'm the one who wrote the report.
Spider jockeys and chicken jockeys are still in the game 1.12.2 java. Some more rare varieties have been removed from the game like Stray spider jockeys and Stray horsemen jockeys.
The way you can obtain them has changed for now due to a bug. Spider jockeys can't spawn naturally but can spawn through mob spawners. Chicken jockeys can't spawn naturally but can from mob spawners as well as baby zombies jumping onto near by chickens. Husk chicken jockeys can't spawn naturally nor from mob spawners but can as baby husk jumping onto near by chickens. Stray spider jockeys can no longer be obtained in the game in 1.12 and same goes for Stray horsemen jockeys
I nicely explain this these videos if you want to see some ingame examples.