There was, in fact, an update to Fences 3.1 which came out on July 22, 2021.

From Stardock's Maintenance and Support Policy:

"Incremental Product Releases

Incremental product releases that do not increase the version number, just the release level, typically contain minor enhancements and fixes. For example: an incremental release is release level 2.19 moving to level 2.20. These incremental releases are available free of charge to registered owners of the version level for which the release is applicable to and can be downloaded from their Stardock account.

New Version Product Releases

New version product releases will be designated by an increase in the top level release version number. For example, when product version 2.xx has a version upgrade the new version number would be 3.xx. New version product releases must be purchased. Discounted upgrade pricing may be available to owners of previous version levels."

Going fom 3.09 to say, 3.9.xx would not be a new version release whereas 4.x would be.

There's another consideration. If your Fences was part of an Object Desktop purchase, the above would be the policy. If you bought Fences as a "stand alone", you would be entitled to continuous updating.

It is unclear to me why you did not receive notifications on updates from the program itself. Where did you purchase your copy of Fences and was it part of Object Desktop or a stand alone. 

You are entitled to a discount if you decide to upgrade to Fences 4. Here's the link.

https://www.stardock.com/products/fences/update

בבקשה, וחג פסח שמח!