Unsupported:
Incognito / Private Browsing Mode: Chrome, Firefox, and Safari do not offer support web push notifications in incognito / private browsing, in order to protect the privacy of their users.
You can see the difference between normal browsing and incognito mode below.
Apple iOS (iPhones & iPads): Apple does not support web push notifications for iOS devices. Apple launched support for service workers but there are still no Push APIs (the feature which will help in sending web push notifications). But rest assured, the feature is currently under development and it’s expected to be released soon.
In-app browsers (Facebook, Instagram, Twitter, etc.): When you open a link on a social media platform like Facebook, the webpage opens on the app's native browser. Since most of these in-app browsers are not based on Chrome WebView, visitors can't subscribe to the store notifications, and so, they don't receive push notifications either.