I don't think this is accurate. Other people may know better, but from my understanding the maintainers of ifconfig realized that to update it to use the new behavior, they would have to end up changing the output. The output of ifconfig is used by a lot of scripts, so changing the output would break a million users, so they decided to write a new tool, rather than add a flag or something to opt into new behavior.

So it's userspace compatibility, not the linux kernel compatibility that lead to a new tool.

(and linux will drop old interfaces if it can be proven that no one uses them, or if they're insecure)