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.
First, my comment says "more likely" so it isn't assuming anything.
Second, if we change "more likely" to "definitely", the assumption is merely that the sentence in question is written with the same command of the English language as the rest of the announcement, i.e. no egregiously redundant words.
That definition does not help the argument that "legacy application" is a more likely meaning than "legacy mysql". In the sentence in question, the application hasn't been migrated yet, so clearly it has not been superseded.
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".
That's a good question, but it's hard for me to objectively say how I'd immediately parse that, due to the close examination I've given that sentence.
You definitely have a point, though. How the phrase "legacy X applications" is interpreted likely depends on whether the context is an announcement by a competitor to X.
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.