In the case of 2.3.7, I think you’ll notice that it is a free release available to everyone regardless of maintenance status; you will see it listed as the most recent Release Channel build available for you.
However, this is not the case in general! As we move towards WM3 and beyond, free upgrades are supplied for a year after purchase. You will always be able to download anything released during that time frame.
The upgrade policy is explained on the upgrade page:
After your first year of upgrades is over, you will no longer receive access to the latest and greatest new releases. You can buy another year of upgrades to continue to be able to upgrade to new editions. Unlike a subscription, your software will continue to function when your maintenance expires. You can download and run any versions you're entitled to, whenever you like.
We no longer will charge for major upgrades, like we did from World Machine 1 to World Machine 2. Instead, while under maintenance you will be eligible to download every major new version or bugfix to World Machine. Even after your maintenance period is over, you will be able to download and continue to use all versions released during the time you were under maintenance. And you can continue to purchase maintenance for as long as you desire.
This is our plan for the future of World Machine development. Instead of taking years between major editions, we’ll simply be continuously improving the program and releasing the changes as soon as they are ready.