unless you deleted them, the older platforms are still there. i have 28, 29 and 30. and i probably still have even older filed away. look, i don't want to mess with your system, but if having what it is that you want is important enough to you, you explore all avenues. but you'll have to do your own bookkeeping as regards the tools you use to build projects. if you're not comfortable switching sdk's, don't do it. or at least don't do it on your production maching.
arp works on my device. maybe you should try searching again, but without "milwifi". that's what got my attention in the first place because it was the wrong tool. also, at one point in time (and in a faraway galaxy), arp was what was used to identify nodes on a network. android is based on a system that supports arp. google and xiaomi are under no obligation to permit its use. xiaomi may have its own ideas about what it allows.
i just goggled around a little searching for xiaomi and /proc/net/arp. somebody posted code similar to what i posted for you. he said
Tested on unrooted Xiaomi Redmi 2 running Android 4.4.4.
so, at one point, arp worked on xiaomi devices.
note: apparently the term ARP (not arp) has a completely different meaning. ironically, ARP is the opposite of arp. if arp tells you who is on your network, ARP is software added to android to
prevent you from scanning your network!!* so, if you decide to google arp and xiaomi, you will get 34,068,252 hits in 0.0002 seconds concerning ARP, not arp... if you want to know about arp (the network utility), you need to say "/proc/net/arp".
(*not quite accurate; it prevents you from loading "old" software onto your newer device, thus bypassing a loophole that would allow you to run /net/proc/arp)