SDRSharp.RdsiPlugin for SDR# NET 5 (.NET 5 Desktop Runtime) saves RDS group data to file in Rdsspy .spy format.
https://airspy.com/download/

Source code (C#, VS 2019 version 16.8) and plugin: https://github.com/OH1GIU-P/SDRSharp.Rdsiplugin-NET5

Copy SDRSharp.Plugin.Rdsi.dll from SDRSharp.Plugin.Rdsi\bin\Release\net5.0-windows\ to SDR# NET 5 folder and add magic line into Plugins.xml file
<add key="RDS" value="SDRSharp.Plugin.Rdsi.RdsiPlugin,SDRSharp.Plugin.Rdsi" />

rds-logger.jpg

rdsspyy.jpg