Do you mean setting the 802.11 mode?
Yep, I had already set 2.4Ghz to 'n' only, and 5Ghz to 'ac' only.
And "Wifi Analyzer" tells me the router is still broadcasting all options: WPS, WPA/WPA2, TKIP/AES.
Seems strange that they would remove the option completely from the new UI. It would have been better to put the option in some "Advanced Users" page, while the general public would use the simpler UI.
My previous router, a D-link DCM-604 from 7 years ago (which this DIR-850L is replacing) gave me full control of the security settings.
Thanks for your reply anyway!