If only they didn't break Google Voice if you used it.
I have stopped giving out my google voice number because I have less confidence in anybody being able to reach me at my GV number ten years from now (vs what is currently my t-mobile number).
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.
Edit: here are the details: https://fi.google.com/about/faq/#plan-and-pricing-extra-high...