Notifications could be hugely valuable for developers who are looking to build feature-rich apps that rely on lots of new web platform features that are not yet supported on all browsers.
Depreciation alerts might help too. I wonder if MDN could build a tool to go through your HTML and JS/TS and notify you of upcoming changes that might impact you.
A sort of reverse-caniuse scanner. I'd love that!
Why? You'll know them anyway from caniuse, given if you want to support a broad range of users you won't want to just support the latest version of browsers anyway.