That's surprising; I've never heard of this happening, and it doesn't sound accurate to me. Calls forwarded by Google Voice to any telephone number are simply PSTN telephone calls. I suspect that the OP simply hadn't received a forwarded call on that DID in 90 days, but rather, was using some other method of receiving calls from GV, such as the plain vanilla XMPP forwarding to the OBi's chat client.
I've had two free CC DIDs since they first offered them. One gets used daily as a GV forwarding number. The other was mainly idle for a couple of years, with just an occasional fax call, until I recently converted it into a forwarding number. I've never been notified about discontinuance.
Moral of the story: just use it or lose it. Seems fair to me...it's free, after all.