When opening on a non-supported browser, for example facebook on an iphone, have a message appear that says if they want to install the app, they need to click on the 3 dots on the bottom right and select open in safari. Do something similar for android.
This is a strong case for keeping this plugin going for much much longer, even if apple adds service worker support. May be possible by detecting if webpage was opened in an iframe.
When opening on a non-supported browser, for example facebook on an iphone, have a message appear that says if they want to install the app, they need to click on the 3 dots on the bottom right and select open in safari. Do something similar for android.