I’ve added that feature in Lunar (https://lunar.fyi/#blackout).

But I guess it might not be worth the price if that’s the only thing you want to do with the app.

While not deactivating the internal screen, dimming the m1 pro until there is not brightness, is the closest thing you can do.

you just close it

That works, but I assume it's not ideal for the airflow.

I’m pretty sure back in the days when you couldn’t do this, people were complaining about their MacBook sleeping when they closed it.

The vents just blast backwards when it’s shut.

I've seen a macbook cook itself doing this, so I'm not sure if it's sufficient

An Intel one, sure. On an M1, first of all, if you're managing to run it at all hot for any length of time, you're doing something extremely impressive, and second of all, I've yet to see my M1 (or hear of anyone's) getting anywhere close to the kind of heat that was considered normal on my last 2 Intel MBPs. I can rest it on my bare legs even when doing things that push the GPU fairly hard; if I'd tried doing that with my last one, I was at the very least asking for severe discomfort.

Unlike Intel-based Macs, M1 runs very cool, and even under heavy all-core load doesn't need more than a slight breeze.

I had no problem on either of my Intel MBP operating them with the lid closed for the two years of the pandemic. I did buy one of those stands that props them up sideways to save on desk space, which also probably helps with cooling as well.