In TPAC 2021, we had a positive feedback on reporting back/forward cache NotRestoredReasons extending Navigation Timing API. e.g. Exposing why a page is not restored from back/forward cache in Navigation Timing API.
link to WG minutes

Here is the explainer: NotRestoredReason Explainer

Our proposal on how to propose NotRestoredReasons is below:

var perfEntries = performance.getEntriesByType("navigation");
for (var i=0; i < perfEntries.length; i++) {
	console.log("= Navigation entry[" + i + "]");
	var p = perfEntries[i];
	// p.notRestoredReason == [{URL:"a.com", Id: "x", blocked: True, reasons:["broadcast channel"], children:[]}]
}

Please let me know if you have any feedback on this. Thanks!