How do you "untrust" a single person's key under this scheme? You would have to visit all of the machines and remove them from the authorized keys file.
Anything signed by the SSH CA will work for logins.
To deal with the “untrust” issue it’s normal for operations with an SSH CA to rely on (very) short-lived certificates, meaning often issued and valid for < 24 hours (it’s configurable, I’ve seen this be as short as 30 minutes).
Smallstep wrote a summary here which is pretty good —
So you want a way to get rid of long-lived SSH certificates, instead authenticating users with your corporate single-sign-on system then issuing them a temporary credential?
And presumably you've got some audit logs, so you know who connected to what, when and why. Perhaps a familiar command line tool, that makes temporary credential rotation easy for users? Perhaps some paperwork to hand to your SOC2 compliance auditors?
I mean, this is sounding a lot like tailscale ssh, teleport, and suchlike...