First of all, that command doesn't actually work: the check is too narrow so as of now it reports every single package as a false positive!
dmbaturin@reki# run show system integrity The following packages don't fit the image creation time build time: 2021-08-22 15:37:00 installed: 2021-08-22 15:40:59 accel-ppp installed: 2021-08-22 15:45:53 acpid installed: 2021-08-22 15:45:52 adduser installed: 2021-08-22 15:45:52 apt-utils installed: 2021-08-22 15:45:52 apt installed: 2021-08-22 15:45:52 aptitude-common
Even if we make the installation date comparison more relaxed, it doesn't solve the root problem: what measure is "not installed at the image build time"?
Any real solution would either check package files in the union mount's read-write dir, or record versions used at build time, or, better, do both.
Finally, my argument for removing it is that its syntax needs to change to avoid messing up with completion. It makes the (very common) show system image command longer to type, while show system integrity is an obscure command not used by anyone in practice.
For the new syntax, my idea is show system package-integrity.