So, the answer to this question is, the Proxmox host will no longer boot.
I just experienced this, although it was on purpose to see what it did. I've had a lab host idle for a while, added the entitlement as I'm serious about properly learning Proxmox VE, applied the updates, saw this message and thought "hmm, did I not enable secure boot?" and allowed it to continue. I performed a few more tasks (added a proper certificate, enabled SMTP, etc.), and then thought I should reboot before trying to add any VMs or containers. Welp, it hasn't come back. The lab is at my office, and I'm at home, so I guess I won't be playing with this again until Monday.
What a stupid problem to have.