This has already been done 🙂
Install this “Options for Twenty Twenty-One” plugin then go to “Customizer – General Options – Inject Sidebar” and choose which pages you would like to show the sidebar on.
Don’t forget to go to “Customizer – Widgets – Sidebar” to add widgets to the sidebar otherwise it won’t be shown!
Oliver
Wow, it looks great.
I’ve just installed your plugin and I am very happy with it.
Please keep up your good work!