Hello Peter Misik
No, Software Assurance is not required to receive extended support for Windows Server. If you purchased Windows Server 2019 Standard, it comes with 5 years of mainstream support and 5 years of extended support, regardless of whether or not you have Software Assurance. When the product reaches its end of life in January, you will still be able to receive extended support until January 2029. I hope this helps!
Reference from:
Windows Server 2019 - Microsoft Lifecycle | Microsoft Learn
Best Regards,
Hania Lian