they are _not_ backwards compatible. That's a big part of the problem. A trailing comma is a syntax error for an SQL engine without support for it
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.