I am trying to change the target framework for my SSIS 2019 script task to 
CodePudding user response：
This is by design!
Since SSIS 2017, you are not able to change the script task/component's target framework since it is forced by the SSIS TargetServerVersion property. As mentioned in the following post, this is by design: Script Task Target Framework - Cannot Change or Save.
Later, In the SSIS extension version 3.2 preview, they mentioned that:
Upgraded .NET framework version to 4.7 for script task/component when the target server version is SQL Server 2019
This means that using SQL Server 2019 as TargetServerVersion will force .NET framework 4.7 as target framework for all script tasks/components.
You can check more information about SQL Server and required .NET framework versions in the following article: