Good question! That aspect is the same as before – the dates for availability are regarding your initial download of the game – the same as now.
If you make sure that you go and opt to download a game before it exists Plus, then you can download it and keep playing it for as long as you’re a subscriber.
Likewise, if you are running short of space, you can delete it from your hard drive and retrieve it from your download list at ANY time – but you have to keep your subscription active.
Playable for as long as you remain a subscriber and accessible in your download list, as long as you downloaded them initially.