The site that has been migrated to the decoupled renderer environment ran into an unexpected issue with the Progress.Sitefinity packages.
As a rule on the site's environments always use latest LTS version of .NET, which is currently .NET 8.
According to Microsoft, it will be supported longer than .NET 9. which is only STS: https://dotnet.microsoft.com/en-us/platform/support/policy/dotnet-core

Progress.Sitefinity packages version 15.3.8500, which come with the Renderer seem to require .NET 9 explicitly. It causes an inconvenience, since the customers will need to build a project based on a non-LTS version of .NET.
Would it be possible to make these packages compatible with .NET 8 or greater versions in the future patches, so that the site can maintain its usual release cadence and not make exceptions for the renderer only? 

For CMS version 15.3 it still requires .NET framework 4.8. Please see the documentation link for more information on Sitefinity CMS and .NET framework compatibility: https://www.progress.com/documentation/sitefinity-cms/reference-sitefinity-cms-and-.net-framework-compatibility

It is not possible to make Progress.Sitefinity packages version 15.3.8500 to be compatible with .NET 8 or greater version because the CMS is not supporting multi targeting for the Asp.Net renderer packages.

The drawbacks far outweigh the benefits of supporting multi-targeting such as  the complexity and maintenance, limited utilization of new features, testing overhead, dependency management challenges, and tooling and IDE support limitations. Given these considerations, the level of complexity introduced by supporting multiple frameworks is not justified by the benefits. It is more efficient and effective to focus on a single framework version to take full advantage of its features, performance improvements, and security patches, while maintaining a simpler and more manageable development process.