install ifupdown2 from proxmox no-subscription repository for debian 64bit
apt install --yes lsb-release wget curl
wget https://enterprise.proxmox.com/debian/proxmox-release-$(lsb_release -sc).gpg --output-document=/etc/apt/trusted.gpg.d/proxmox-release-$(lsb_release -sc).gpg
echo "deb [arch=amd64 signedby=/etc/apt/trusted.gpg.d/proxmox-release-$(lsb_release -sc).gpg] http://download.proxmox.com/debian/pve $(lsb_release -sc) pve-no-subscription" > /etc/apt/sources.list.d/pve.list
echo "Package: *\nPin: origin \"download.proxmox.com\"\nPin-Priority: 50\n\nPackage: ifupdown2\nPin: origin \"download.proxmox.com\"\nPin-Priority: 500" > /etc/apt/preferences.d/pve
apt update
apt list --upgradeable
apt install --yes ethtool bridge-utils python3-mako ifupdown2