For the sake of completeness, pve8to9 didn't show me the FAILED item prior to update, only after the update.
I was worried about the server not rebooting because of the warning, but it still rebooted fine.
And I followed the pve8to9 instructions...
so I did :
apt install systemd-boot-efi systemd-boot-tools
apt remove systemd-boot
apt purge systemd-boot
It told me systemd-boot-efi and systemd-boot-tools are already installed...
Rebooted, all seems fine, `pve8to9` is all green.
Other people have done it too: https://forum.proxmox.com/threads/whats-the-expected-content-of-etc-initramfs-post-update-d-systemd-boot.171377/post-798468 (and some got in trouble cause they did not do it).
I think I'm in the same boat...
After performing the update & rebooting here's where I'm at :
root@pve:~# pveversion
pve-manager/9.0.6/49c767b70aeb6648 (running kernel: 6.14.11-1-pve)
root@pve:~# proxmox-boot-tool status
Re-executing...