People have been worrying that Google would shut down GrandCentral/Voice for years, though, ever since Google acquired it nearly a decade ago. So far those worries haven't been realized. Google Voice has even received major mobile and browser upgrades last year, with regular updates since then: https://www.androidpolice.com/2017/01/23/google-resurrects-v...
So it appears Project Fi doesn't work with Google Voice not because the latter is being abandoned, but because Project Fi is based in part on Google Voice so you can't use both at the same time. Google Fiber home phone service also appears to leverage Google Voice infrastructure: https://www.androidpolice.com/2016/03/29/google-fiber-rolls-...
I would probably suggest porting your Google Voice number to a secondary Google account if you don't want to make your Google Voice number your Project Fi number instead.
Project Fi shares a lot of the same features Google Voice has: https://support.google.com/fi/answer/6159953
So it appears Project Fi doesn't work with Google Voice not because the latter is being abandoned, but because Project Fi is based in part on Google Voice so you can't use both at the same time. Google Fiber home phone service also appears to leverage Google Voice infrastructure: https://www.androidpolice.com/2016/03/29/google-fiber-rolls-...
I would probably suggest porting your Google Voice number to a secondary Google account if you don't want to make your Google Voice number your Project Fi number instead.