It's still possible to port-out your Google Voice number for a fee. I wouldn't rely on keeping Google Voice after Google kills the legacy accounts. If you have to, port out to a different SMS service (you could port back in to consumer Gmail if you really wanted to).

Back when legacy accounts weren't legacy, Google Voice was free for G Suite (I think they are now called "unmanaged") and Google is trying to get rid of those accounts in favor of newer paid "managed" accounts.