So, after upgrading Proxmox from version 6.x to 7.x, the network was suddenly gone. Turns out, the network service wasn’t started anymore.

According to systemctl, it was disabled. After some googling, it seemed to be that this was because I was doing was the Debian docs were saying, performing an apt purge, deleting the ifupdown configuration. This seemingly disabled networking.service.

So I symlinked the unit into /etc/systemd/system/, rebooted and everything was fine again. Thank godness for physical access to hardware.