Controlling external displays via DDC was never supported, and only works even on Intel Macs via a "private," undocumented API. It's neither appalling nor even surprising that this wasn't prioritized for the M1 kernel, and it is likewise reasonable that some more reverse engineering would be required.

I know it’s not how it’s supposed to work, but I think once an API is used in real world, and sees useful applications that some users rely on it day to day, it should be seen as “public”.

I never heard of Lunar before but completely understand the use case, all the more so it doesn’t seem Apple is interested in filling that gap outside of its pro display.

There's no guarantee that Apple even knew that this API was being used.

From other comments, it would appear that they did used to support this and only cut it out since Catalina?

It works through at least Big Sur on Intel. Maybe it was previously documented and they pulled the docs around Catalina's release, but this would be the first I've heard of that in at least a year of active interest in the topic, and that makes it difficult to credit on pure assertion. There are almost a hundred comments in this thread, and it's close to midnight here; do you mind linking those to which you refer?

It would be nice if it just worked, however I’d rather MacOs folks work to fill out the complete functionality if Spaces.

Spaces, the multiple desktops, single monitor feature of MacOs has been neglected for too long.