I guess a bunch of "roll your own X.509 validation"-logic will have that bug, but to exploit it you need a misbehaving CA to issue you such a cert (i.e. low likelihood)
Well, I'm on a Mac and it 100% has been implemented for me -- I tried to figure out how to turn it off as soon as I first saw it, went to preferences, found the checkbox. I didn't realize that hasn't been rolled out to everybody.
Downloading things from the AIA fields would mean triggering HTTP/HTTPS requests to an untrusted URL from a certificate you haven't verified - not a good idea. What firefox does is cache intermediates that it has seen elsewhere, the windows TLS stack can fetch additional certs from windows update on-demand (and actually starts with only a small bundle of trusted roots). There is no good solution for incomplete chains other than getting the sites fixed (or using a provider like cloudflare that solves it for them).
1. A SYN will receive a SYN-ACK or a RST
2. A host from my machine is the same as from your machine
3. An IP from my machine is the same as from your machine
It was also possible in earlier browsers by using hidden frames. User interaction or a timer would trigger, then you load the frame (which contained dynamically generated js) and then call the javascript you'd just loaded.
You can't get perfection, but you can do things like ask for them to document the controls they implement and to evidence that they have operated them appropriately via an audit like SOC2.