Doing this only provides deniability in public. If this was brought to a court there are enough server logs to build out if that DKIM record was valid along with a number of DNS history providers.
My case was a family court dispute where one parent forged an email from the other parent about the child's care. It was a pretty wild case. After that, I was pretty convinced some people are just evil.
Yea, stuff with kids involved sometimes makes it SUPER unambiguous. Some people are very clearly willing to do anything to anyone, and are doing so as much as possible.
Most people are not aware of what it means to be sociopath/psychopath/Anti-social personality disorder... Or how common it is in the population (a few percentage points seems reasonably accepted) - Why is this information not made general knowledge?
I would argue it should be taught in schools.
It's a basic life skill, to be aware of what it means and how common it is. How do you function in society being oblivious of these facts? Basically by perennially becoming a victim to these, or by becoming bitter.
Just knowing about it is half the battle.
I would also add other dark personality types with potential to cause you real harm to the list - BPD and NPD.
School yourself, save yourself a whole lot of bad surprises. Over a lifetime being aware of the existence of these means you are on the lookout for warning signs, and you learn to get better at picking up on warning signs faster.
This isn't about evidence. In a court case, discovery and court orders can authenticate email messages even if providers publish their keys; the provider will have a record of having verified the email when it was received. But ATO attackers will not have access to those records.