You just get the warning like every users of the app, no matter you will be affected or not (I mean directly). So this app will still work on your GrapheneOS device using third party store or direct apk install. The problem is not here, the problem is once only users with custom ROM and third party store will continue to use these app, it will come down to a fraction of the users and what will motivate the devs to continue support of these apps ? Many small open source app will die for lack of usage and this is affecting everyone indeed.