Is it real?

Yes, it’s real and I’ve done it. It’s called the First Cliff Walk. You take the First gondola all the way to the top. Then walk uphill a bit. Then you walk along the walkway and across a suspension bridge. The thrill walk is free, and the gondola ride is covered by the Berner Oberland pass. It is discounted with the Swiss Travel pass. I believe it is only open certain months - not in winter.

There are other adventures you can do at this same mountain. We did the mountain carts in June. Expect about an hour wait for each activity in the summer. https://www.jungfrau.ch/en-gb/grindelwaldfirst/adventure-package/

This?
https://www.jungfrau.ch/de-ch/grindelwaldfirst/first-cliff-walk-by-tissot/

The First Cliff Walk is open in Winter, weather permitting.

There is another one at Brig (on the Schilthorn Bahn) that is far more spectacular in my opinion, but that currently is impacted by the construction of the new Schilthorn cable car.

Are the improvements on the Schilthornbahn installing a whole new cableway or just maintaining or getting new vehicles for the old one?

Wow, thanks good to know. Switzerland just moved up higher on my travel list!

Are the improvements on the Schilthornbahn installing a whole new
cableway or just maintaining or getting new vehicles for the old one?

All new cableway. You can read and watch all about it here:

https://schilthornbahn20xx.ch/en/Welcome

https://schilthornbahn20xx.ch/en/Info/Planning_Schedule

https://schilthornbahn20xx.ch/en/Infos/Videos