It can be long discussion, because there is no single solution around this, so, to cut the story short, I will tell where I stuck on my way.
It is AndroidAuto (media centre in car on raspi) project I'm currently working on, and I'm stuck on effectively powering raspberry and safely shutting it down when ignition is on/off.
Initially, I went with mausberry solution circuit, that takes 12v const/ignition/ground from car battery on input, takes care of voltage conversion and triggers safe shutdown in the event of ignition goes off. But mausberry didn't supply the circuit, so when I fed up to wait I decided to go UPS route.
I use, one of them, Geekworm (geekworm.com) UPS HATs (+ separate unit for 12v-to-5v conversion) that goes over Raspberry GPIO, takes ignition as power source, charges its battery and in parallel powers Raspberry while ignition is on. When ignition goes off, Raspberry stays on and battery on UPS starts discharging.
At this point I need to trigger safe shutdown, so that no electronics draining my car battery.
Geekworm UPS HAT, also, have annoying switch that has to be pressed to give power to Raspberry, which I don't need as I want to start Raspberry when ignition turn on. This is possible to overcome by just using a micro usb splitter that will go into UPS and Raspberry from ignition power source. That is stupid solution to my view, so if anyone can help with more elegant solution, I'd appreciate.