"upgrades have been deferred due to phasing"

If you are using Ubuntu 24.04, you are likely to come across the following message while updating your system in the terminal:

The following upgrades have been deferred due to phasing

For example:

martin@church1e$ sudo apt update && sudo apt upgrade -y
[sudo] password for martin:
Hit:1 http://security.ubuntu.com/ubuntu noble-security InRelease
Hit:2 http://gb.archive.ubuntu.com/ubuntu noble InRelease
Hit:3 http://gb.archive.ubuntu.com/ubuntu noble-updates InRelease
Hit:4 http://gb.archive.ubuntu.com/ubuntu noble-backports InRelease
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
2 packages can be upgraded. Run 'apt list --upgradable' to see them.
Fetched 89.7 kB in 21s (4,340 B/s)
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Calculating upgrade... Done
The following upgrades have been deferred due to phasing:
  dmsetup libdevmapper1.02.1
0 to upgrade, 0 to newly install, 0 to remove and 2 not to upgrade.

DON'T PANIC ... It's not an error it's just a message that informs you that the package(s) in question have an update but it'll be rolled out in phases. In other words, if the upgraded package(s) causes regression, the updates are rolled back to lessen any greater impact.

Thinking out loud, I'm sure this is a reworded version of the message that was seen in previous versions of Ubuntu

following packages have been kept back

You could try running the update process again in a few days to see if the package(s) have been updated.