Ubuntu 21.04 To Expand The Use Of Phased Package Updates
Going back a number of years in Ubuntu has been Phased Updates that wired into Update Manager has led to the gradual rollout of new stable release updates over a period of about two days. This has been done gradually to ensure that no regressions or potential big problems hit all Ubuntu users at once by over the course of many hours exposing more Ubuntu users to these updates.
With the phased updates having been handled by update-manager, it hasn't been implemented universally by all Ubuntu flavors or means of updating packages. But with Ubuntu 21.04, APT now supports phased updates itself.
With APT now supporting phased updates, this means the slow roll-out will happen for Ubuntu Server, containers, Ubuntu on Raspberry Pi, and other variants. Ubuntu in chroots or those dropping the machine-id option for APT will still see the "classic" non-phased behavior. Thus users can still control whether they want to see the phased updates behavior or manipulate their system state to match (or not) the same timing as multiple systems under their control.
Those curious about this more widespread use of phased updates beginning with Ubuntu 21.04 can find the details via this Ubuntu thread.