If you are browsing the web with a 10 year old browser you are opening yourself up to a ton of security bugs. Whether json responses contain a while loop or not isn't going to make a difference.

The reason Google and Facebook keep this kind of stuff around is because it's there and doesn't hurt to keep it. There's a slight chance it will provide some protection if a similar attack vector is discovered.

But aren't you saying this is an already existing attack vector then?? Why try to find a similar one if you knew you could just get an older browser version and use this one? Is that not a good enough reason to be prepared for it?

As sagethesagesage said [1], you're protecting the user from having their browser pass the user's data from your site to a malicious site. The attacker shouldn't be able to make the user run an old, vulnerable browser.

[1] https://news.ycombinator.com/item?id=14282532

It's more that people who use those browsers are being protected. It's not that those browsers can poke security holes in the site, they're just vulnerable to losing their own data.