He hasn't been officially confirmed, he's just highly likely. In addition to being in the background of the SF arena (as you mentioned), This image taken from a screengrab of a developer's monitor during a development video appears to be him performing a fatality on Sub-Zero (wearing the same outfit he's wearing in the SF arena)...

https://pbs.twimg.com/media/DxPNnhYXcAEBVRi.jpg

Many people just have a loose grasp of what the word "confirmed" actually means and tend to jump the gun. Still, his odds seem very strong, based on that shot. He's a pretty safe bet at this point.

He's been semi-confirmed in an interview with Steve Beran: https://www.gamezone.com/originals/interview-mortal-kombat-11/

From the end of the article:

Like Johnny Cage, that’d be a great one to see.

''You get the idea, Johnny Cage is a great example of that where yeah he’s fifty-something now, and you know, matching him up with twenty-year-old douchebag Johnny is gonna be interesting.''

This interview, the fatality, the roster as it's looking already, they will not leave out Johnny. Jax on the other hand is less likely..This is the likely the roster, though Noob and Rain are still up in the air:

• 1. Scorpion
• 2. Sub
• 3. Raiden
• 4. Sonya
• 5. Baraka
• 6. Skarlett
• 7. Erron Black
• 8. Kano
• 9. Kabal
• 10. Dvorah
• 11. Jacqui
• 12. Kotal
• 13. Cassie
• 14. Jade
• 15. Kitana
• 16. Johnny Cage
• 17. Kung Lao
• 18. Liu Kang
• 19. Frost
• 20. Shang Tsung
• 21. Rain
• 22. Noob
• 23. NEW KOL (The Kollector)
• 24. NEW TER (new girl, supposedly really badass)
• 25. NEW Geras
• 26. DLC Shao Kahn

Geras = CET according to inside sources (not mine).

First I'm seeing it, but fair point.

Again, I don't doubt his odds are strong, He's pretty much all but "officially confirmed" (trademark) at his point.

This is the first time seeing this as well, I didn't know that he was shown further than just the background cameo.

Thank you Barakall for sharing this information, I appreciate it! :)