Just to expand on that, I always say there are three audiences for the commit message:
1. The reviewer
2. Someone scanning commit logs for patches of interest
3. Archaeologists
Archaeologists are people a year, five years, 10 years down the road who are trying to figure out how the code got to be the way that it is, so they can feel confident changing it. (See also "Chesterton's Fence"[1].) Those people should not have to dig out the conversation from some random PR to figure out what's going on.
A fairly common occurrence when looking at old commits is that any URLs mentioned are now 404s, and ticket numbers are for an issue tracker that no longer exists.
[1] https://github.com/obsproject/obs-studio/pull/5875