This is not the case as of yesterday at least in some corner of their order system; when I was getting through their ordering system at the bottom of the page where you were signing up for the phone line there was a checkbox to allow you to request not to be served via digital voice.
Indeed that's a new option. I don't know why they didn't offer that a couple years ago when they launched Digital Voice.
Waiting till 6 months before the service is withdrawn from sale is somewhat pointless.
There's still nothing stopping them changing it to Digital Voice at any point though unfortunately.
Their contract isn't specific about how they deliver their voice.
They wrote to my mum a couple times about moving the line to Digital Voice and warning if she had an alarm or similar device that required dial tone to contact them to block the move.
They did that twice, the 3rd time she received a package with a Digital handset and a letter informing her of the move date.