I can’t talk about future releases. As a general rule, though, we don’t break backward compatibility, Apple does.
That is, over time the toolkits that Apple supplies and Scrivener depends on evolve. Eventually, it becomes impossible to support both the old version and the current version of Mac OS with the same piece of code.
A recent example of this was Apple discontinuing support for 32-bit applications starting with Catalina in 2019. Scrivener 2 is a 32-bit application, so it would be impossible to bring it forward to Catalina+ without a complete rewrite. Which is essentially what Scrivener 3 was, and Scrivener 3 was released in 2017. So our answer to users looking to upgrade to Catalina+ was and is: install Scrivener 3.
But we also have the same problem in reverse. For Scrivener 3 (a 64-bit application) to be backward compatible to 32-bit versions of Mac OS would essentially require two versions of the application, one using versions of Apple’s toolkits that are so old that Apple doesn’t support them anymore. Double the development time, more than double the support issues, for a small and shrinking fraction of our user base. So our answer to those users has been “We will continue to support Scrivener 2 for as long as we have users running it, but if you want Scrivener 3 you’ll need to upgrade your hardware.”
So my advice to users generally is to be sure to understand the migration path for software that is important to you. The time to learn that Scrivener 2 won’t work with Catalina is before you install Catalina, and especially before you buy new hardware that will only run Catalina. And don’t take either of those steps in the middle of a deadline crunch.
Also, be aware that you don’t have to run the latest version of Scrivener (or Scapple). Its feature set is fairly mature at this point, so many of the changes between versions are simply compatibility updates to support the latest version of Mac OS. If you’re using Mojave, you don’t need to care about Ventura compatibility. One of the reasons why we continue to offer downloads for older versions is to allow people to back-grade if they (or we) discover an issue with the newer version.