Auto brightness and True Tone have two completely different goals. Same with night shift.
Night shift is basically a accessibility feature. Helps to limit blue light exposure. True Tone changes the white point to look more natural in the room it’s in. Like how a white piece of paper looks different depending on the lighting. Don’t think I need to explain auto brightness.
True Tone doesn’t just make the screen yellow/warm. It will even go slightly blue with the correct lighting.
Because I got a new phone and lost data in the process, let's say I've done a lot of backups in the last few days...
With that said during the setup process when Apple ask if you want to keep true tone on (but it really isn't an option to turn off during the setup). It's blatantly obvious that Apple wants you to use true tone; because when you hit the button to see what the screen would look like with it off, it's hilariously more blue/cool than it ever is with the phone fully setup and true tone shut off. I never questioned what night shift did, but what I'm saying about true tone is that the screen looks a lot less vibrant with it on, and auto-brightness does a more than adequate enough job to balance your screen out depending on your lighting conditions.
So, again if my screen is more vibrant with true tone off and auto-brightness makes the necessary lighting transitions for me... Again, what is the point of True Tone!? IMHO, it's Apple trying to make the screen as LCD'ish as possible and not let OLED shine to its fullest potential.
I maintain that True Tone is trash!!!