Distance is the magic factor for all calibers, doesn't matter what you're shooting, theres a limit on all of them. And ill stick to real world, real conditions that i've experienced and witnessed. FWIW I only gave 1 example of witnessing the 243 flatten deer. Ive watched it probably 25+ times in my life between friends, family and my own experiences. Same with 270, 308, 7mm-08 etc etc. The point is that everyone thinks they need a 300 win mag to kill a deer. Again, bigger calibers don't make up for poor shot placement.
I think we can all agree that 1000ft/lbs of energy is a good number needed to ethically flatten a deer. Realistically its probably closer to 750-800 ft/lbs but for the benefit of the doubt, 1000ft/lbs will be the number.
A 6mm 103gr ELD-X at 2900 fps holds 1000ft/lbs of energy out to 500-550 yards. To most people thats a long shot and 85% of hunters will never shoot an animal that far away. That being said, you still need a GOOD bullet that has good terminal performance. A "long range" bullet at 75 yards wont perform as well as a bonded/soft point (interlock, accubond etc) at the same range. And again, vice versa, a long range bullet will perform better over an accubond, interlock etc,. at 500 yards once it slows down a bit.
The summary is that most people will agree that a 243 is just fine for deer with the proper bullet choice and placing that bullet where it needs to be. Those who don't agree either used the wrong bullet or decided a texas heart shot was sufficient enough and ended up losing the animal.