It is covered in their official documentation here. Specifically the big blue box at the top of the article. Note that what is technically going out of support is the server OS. Since the server OS won't be supported anymore then software that runs on it won't be formally supported either.
Going out of support means that MS will no longer provide support should you run into issues. They will no longer worry about the unsupported OS version when adding new features so a future update may stop working on unsupported OSes or not install at all. This goes for the OS as well because Server 2019 is end of main stream support already.
So, you can continue to use the software after the given date. It won't just stop working. However future updates may start using features that it doesn't support. Nor will you receive any support if things stop working.