Yep, you do get to keep the game key from Patreon! You'll even get future updates as they come out! Some people are abusing this and it definitely sucks for me and the game's development, but eh, them's the breaks.
I honestly appreciate people telling me that they're cancelling their membership infinitely more than the emotion-less list of notifications Patreon serves me to let me know how many people cancelled theirs. That list is gonna be the death of my sanity. You don't appreciate Patreon's Exit Surveys until you're running your own Patreon.
But hey, feel free to come back whenever you feel like it. ;)