You read that right. Google Voice phone numbers expire just like any other phone number from the main 3 carriers like Verizon, AT&T and T-Mobile or your specific country main carriers.
You have to have some form of activity on your Google Voice phone number at least once every 90 days, meaning that you should send at least one outgoing text or make an outgoing phone call every 3 months, but some people have reported getting deactivated in less than 60 days.
So if you for example are using Google Voice to get a phone number after you’ve physically left the US, remember to use your Google Voice phone number every now and then or risk losing it.