It is in the F.A.Q.:
Do you need to pay again for future versions of Zorin OS Pro?
The purchase will give you access to Zorin OS 17 Pro and future updates within the 17 release series.
Zorin OS 18 Pro will need to be purchased separately when it will be released in the future. We will continue to support Zorin OS 17 with software updates until at least June 2027, and you will be able to download and use Zorin OS 17 Pro for life.
But I think you make a good point: Perhaps it should be On The Purchase Page.
If a new edition is likely to soon be released; it could include a disclaimer to cover that, as well.