A bit of Ubuntu Server network config history:

Ubuntu Server, like a lot of distros, stored network configurations under a single filename: /etc/network/interfaces. Of course, starting and stopping them was a bit different.

With version 17.04, they changed it to netplan. Both IPv4 and IPv6 are handled there, and configs were stored under the /etc/netplan directory. Starting and stopping network interfaces changed as well.

With version 19.04, they changed it again to systemd-network. They helpfully migrated the configs to the appropriate /etc/systemd/network directory and had 'netplan' in the filenames. (Uninstalling netplan would be a bad idea here; you need renderer: networkd in a netplan .yaml file instead.)

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That same year, I spent over 30 minutes attempting to migrate IPv4 over (IPv6 was successfully migrated and was disabled in netplan) but eventually gave up, mainly because I knew no better of the systemd-network formatting back then. So this pretty jank network interface setup that stayed for six years is a result of the double migration of interfaces.