Well, you can use them if you want to, its not like they dont work
In general: frontend code (also CSS) change way more often than backend code and there is no clear deprecation/change timeline. But that’s totally fine since everyone rolls his own stuff. It’s not like you can implement anything useful that isnt totally outdated in a year.
As for dialogs/notifications: dialogs (modals) are usually bad UX and you absolutely shouldn’t use them. The notification system itself is so minimal that it probably won’t work for you. And placing a modal/notification in your app is close to 0 work.
TL;DR: it’s a non issue. You just assume its one