It can disappear and reappear, but I wouldn’t recommend it as go-to behaviour.
What if you’re reading an article on Medium’s iOS app and you notice a badge or coloured symbol next to another tab’s icon? You must have got a notification.
You stop scrolling altogether and want to instantly go back to another tab. But oh no! the tab bar is nowhere to be seen because you haven’t begun scrolling back to the top.
Navigation should be available at all times, not when we anticipate a user needs it.