vscode: [ts] Experimental support for decorators is a feature that is subject to change

You've opened the editor on the src folder. For vscode's language service to find your tsconfig, you need to open the editor on the true root of the project holding your config.

Opened on root folder:

Opened on src folder:

You can set

"javascript.implicitProjectConfig.experimentalDecorators": true

in your vscode setting.