Counter-Example: I ran an air without a monitor connected for years using caffeine, worked perfectly for me..
I did for years too, but newer MacBooks no longer allow running with lid-closed unless connected to a monitor, I was disappointed to recently learn this.
If I’m wrong, someone tell me how to do it! On an M4 MacBook Air running latest OSX release.
I don’t mean this to sound like I’m being a jerk, but why would I want my MacBook to be awake with the lid closed? If I want it to be awake doing something, I leave the lid open and let the screen sleep. Maybe I’ve been using a Mac too long, but lid closed means sleep to me. I’ll even do that when I have it plugged into a monitor: close the lid to make it sleep.
You can't, really. Almost everyone resorts to buying an HDMI dongle to fake a display. Apple solved the problem at such a low level, the flexibility to run something in clamshell mode is broken, even when using caffeine/amphetamine/etc etc etc.
So, tradeoffs. They made their laptops go to sleep very well, but broke functionality in the process. You can argue it's a good tradeoff, just acknowledge that there WAS a tradeoff made.