It's two years later, and I'm still waiting for those improvements in Jira. Until then Jira is a great case study of how not to do things in JavaScript, it's slow, it's inconsistent and I doubt the accessibility is very good.
(It's also a great case study of how it doesn't matter how good or bad your app is, the success of your business doesn't depend on that)
We're definitely very slow on load at Atlassian, but our accessibility is actually really good. One benefit of a good component library (in our case React) is it allows mapping of accessibility labels fairly easily. Jira should be fully navigable with just a keyboard and headphones.
(It's also a great case study of how it doesn't matter how good or bad your app is, the success of your business doesn't depend on that)