Wireless-Extensions (WE or Wext) are the extensions added to the kernel circa 1997 by Jean Tourrilhes. We don't document WE as Jean already has documentation for this on his page so what we do do here is document things you should know about WE if you didn't before.
No it is not. Only bug fixes are being accepted for WE.
WEs are based on ioctl() and although ioctl() has been used and still is being used as a standard transport for communication between user ←→ kernelspace new transports are being preferred for several reasons.
From Linux Device Drivers - 3rd Edition:
int ioctl(int fd, unsigned long cmd, ...);
It also states:
No. nl80211 is a complete re-design of how wireless settings work and more clearly defines the semantics of each command (group).
No, while cfg80211 still has some backward compatibility support, and a few ancient drivers still support only wireless extensions, everything else has long moved on and doesn't support anything but the most basic features with wireless extensions.