Honestly, one of the worst parts of the Linux community is people trying to force 30 years old tools in systems built around systemd. If you want to use that old stuff then don't install the modern replacement, find a different distro built around that ideal instead.

I remember a post on serverfault or askubuntu about disabling DHCP default gateway but keeping DHCP address assignment on a tap interface, and bring it up at boot, and the accepted answers was "configuring DHCP in networkd/NetworkManager systems is almost impossible, here's how I did it" and it's three pages of cobbling together ifconfig and netplan with startup scripts, that work by pure chance.

Wanna see the actual full networkd configuration for that?

# /etc/systemd/network/tap0.network
[Match]
Name=tap0

[Network]
DHCP=ipv4

[DHCPv4]
UseRoutes=false

That's it, all that the post asked for is handled by six lines. "How do I discover that?" you may ask, because if it's three lines, but I have to dig for hours before finding it than it's not that useful. Simple, I go on the systemd documentation for .network files and search for DHCP. And this is a niche use case, the basic usage is readily available on the arch wiki as with anything else. Note, this does nothing for IPv6, and the interface will have IPv6 route configured, but this wasn't relevant to the post, and my home's IPv6 layout is "peculiar" so I have omitted it here.