Earlier quoted context omitted.
delete indexedDB; in userjs is all it takes in Chrome to disable indexedDB permanently
It is still possible for websites to access indexedDB by using Web Workers: new Worker("data:application/javascript,console.log('indexedDB: ', indexedDB)") This works since userscripts only run in top-level websites and frames, but the above code runs JS code in a seperate thread with no attached DOM.
if ('serviceWorker' in window.navigator) navigator.serviceWorker.register = () => new Promise( function(resolve, reject) {} );
disables webworkers from ever working.