I double checked the Linux Kernel documentation, and it’s documented as disabled by default. I recall @Mario_Limonciello may have mentioned that this may be changing to enabled in a kernel version (6.8/6.9?). Is this documentation out of date then?
abmlevel (uint)
Override the default ABM (Adaptive Backlight Management) level used for DC enabled hardware. Requires DMCU to be supported and loaded. Valid levels are 0-4. A value of 0 indicates that ABM should be disabled by default. Values 1-4 control the maximum allowable brightness reduction via the ABM algorithm, with 1 being the least reduction and 4 being the most reduction.
Defaults to -1, or disabled. Userspace can only override this level after boot if it’s set to auto.
This doc also appears to indicate that both 0 and -1 represent disabled.
https://docs.kernel.org/gpu/amdgpu/module-parameters.html
Edit: I believe this is the thread I was referencing | Adaptive Backlight Management (ABM) - #54 by Mario_Limonciello