[pbreit]

From Google: "denoting software or hardware that has been superseded but is difficult to replace because of its wide use"

Maybe not a perfect word but very easy to understand in the context.

[brlewis]

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 In the sentence in question, the application hasn't been migrated yet, so clearly it has not been superseded.