.arpa is in use, but only as an internal detail for reverse name resolution (i.e. looking up the PTR record for 1.2.3.4 queries the server 4.3.2.1.in-addr.arpa)
Which also yields one of the more interesting backronyms of the internet, as .arpa today is understood to mean "Address and Routing Parameter Area" as opposed to the Advanced Research Projects Agency that was involved in the early internet (ARPANET).