I don't think any of us really knows, but I would assume the worst which is that if you let it lapse, all your Matched data will be gone.
In reality, and this is just a guess, it's possible that Apple will allow for a small window in case the subscription lapses and you didn't want that to happen (ie. your credit card expired and you forgot to change it). But even if they do this, it's likely a small window before they purge your data completely. In other words, I wouldn't risk it - just back everything up to a local drive to play it safe.
I hope everyone is doing that because trusting all your songs to iTunes Match only is really not the best idea, imo. Local backups are still critical if you really care about your data.