I read debian/patches/netplan/0002-netplan-make-use-of-libnetplan-for-YAML-backend.patch and it dont looks like it is possible to disable. Looks like the only option is to recompile the whole NM package. Why you have not gone a clean way with a seperate plugin which we can select to install.
plugin: keyfile or keyfile-netplan or something like it.
I really see no more comfort in netplan and so don’t like to be forced to it. For me netplan was always just an option - but now things changing which makes me not happy.
From your webpage:
Netplan has established itself as the proven network stack across all variants of Ubuntu – Desktop, Server, Cloud, or Embedded. It has been the default stack across many Ubuntu LTS releases, serving millions of users over the years.
But you must also see, that not everybody used netplan - I am a Ubuntu fan for long time and also installed over 500 server and desktop systems with it. I was every time free to use what I want and newer went to netplan. I used systemd on servers and NM on desktop systems.
For sure I am not the only one who liked to have a choice.