I know MIPS was still common for low-end 802.11ac routers/APs (eg. MT7621-based devices). Is it being used by anyone for systems designed around newer standards (802.11ax/WiFi 6/6E, multi-gig Ethernet) or has the market finally shifted over completely to ARM?
MIPS, yes, but not big endian. Last time I started building binaries for my (utter junk) EdgeRouter-X I ran into issues with mipsel stuff not being well supported. If Marvell and MediaTek aren't putting out big endian MIPS stuff I'd be surprised if anyone was.
I guess it's not too surprising somebody paired newer radios with the MT7621 SoC. I see now that MediaTek even lists that application on the product page, so they're probably not going to retire that processor anytime soon: https://www.mediatek.com/products/home-networking/mt7621