30+years ago I ran a wire from the coil (can’t remember which HT/LT) via a on/off switch to a earthing point, effectively immobilising the vehicle. Q: is this still possible to do on modern day vehicles? If so which lead do to attach it to?
NO, do not try to earth any of the coil(s) on a modern car - you could easily damage any one of several control units.
However, you could consider putting a switch "in-line" with the fuel pump feed - much safer and will prevent it starting quite neatly.
As Solar Mike states, dont earth your coil(s). Easier yet, most modern cars have fuses which provide power to the coils and/or fuel injectors. Pull either one and the car won't start. Pulling your fuel injector fuse is better, because you won't wash your cylinders during the cranking process.