Yep. Been a long requested feature. One of the reasons why the post states: "This feature also removes the last significant barrier to migrating legacy MySQL applications to PostgreSQL."
What do they mean by "legacy" in this context? INSERT ... ODKU is not a legacy feature of MySQL, it's a currently supported first-class feature of the database, nor is MySQL itself a "legacy" database.
I think they're referring to mysql. The sentence would have worked just as well without the word "legacy". I say this as someone who prefers PostgreSQL.
No, they are referring to an application that is being moved from MySQL to PostgreSQL. The application is "legacy" in that it is an older version and the new version is "current".
Due to the English language however there can be some debate as to what they meant. In this case "legacy" is most likely meant to describe the "MySQL application" not "MySQL" itself.
A meaning that adds something to a sentence is a more likely meaning than one that adds nothing to a sentence. If you take "legacy" to mean "being migrated from" then the sentence becomes
This feature also removes the last significant barrier to migrating being-migrated-from MySQL applications to PostgreSQL.
It's more likely that if "being migrated from" was the intended meaning, they would have simply left the word out.
Your comment assumes the author of the release notes has perfect command of the English language and they thought through in detail what the word "legacy" would mean in this context.
Replace MySQL with Django. Do you assume Django itself legacy, or the Django based application? I parse it correctly as "a legacy application that uses MySQL".