Who is writing SQL queries expecting them to fail because of this reason?
By your definition, "backwards compatible" doesn't mean anything. Literally everything will be a breaking change if you define "backwards compatible" like this.
But they never ran fine on engines that didn't support trailing commas in the first place :/
What you're calling "forwards compatible" is what I call "backwards compatible". Frankly, I suspect most people expect "backwards compatible" stuff to work like this.
Is this distinction useful in any way?
It's backwards compatible from the perspective of the database - the db will continue to support queries from older versions.
I believe you're speaking from different perspectives.
If you are trying to contribute, you will use the phrase backward compatible as we always do: my old code will still run after the change.
If you are trying to be pedantic, you will choose a new way of looking at it: My new code will not work if you go backward in time.
I don't care. I will swear to never write a dangling comma for any engine that doesn't support it. Also, I will swear to never use an engine that doesn't support it if once that does exists... Just as I did with Javascript.