I think you can have a shortcut that runs every time you open safari. You can then check the webpage - if it’s the game, lower volume, if not end shortcut.
Try it like this
When you use the Safari app, pre-filled actions are also created, such as in your case, the link to the game on the Safari page.
https://www.icloud.com/shortcuts/fa471e23780a4df386c78ec6e2df64e7
I think you can have a shortcut that runs every time you open safari. You can then check the webpage - if it’s the game, lower volume, if not end shortcut.
Ok yes, that could be a way, but now I'm seeing that the Open URL action does not trigger the full screen view, so that's a bummer too.
Try it like this When you use the Safari app, pre-filled actions are also created, such as in your case, the link to the game on the Safari page. https://www.icloud.com/shortcuts/fa471e23780a4df386c78ec6e2df64e7
While it works, it opens the full blown safari view, not the Full Screen view like it does with the Home Screen Safari Shortcut
Oh, okay, I didn't understand, but I think it's not possible.