> So what exactly are they supposed to say?

Several options, pretty much all of them involve being actually cooperative rather then intentionally unhelpful. If Bill was part of some other team, point to that team or its leader.

If he was in your team, you or leader can ask about what the person wants and move from there. Maybe you can actually answer the question. Maybe the proper reaction involves raising jira ticket. Maybe the answer is "we are probably not going to do that anymore". It all depends on what the person who came with the question wants.

> But it doesn't change that Bill was the person who was responsible, and now is gone.

The other people are still there. And the team IS responsible for X. And without doubt, they are fully responsible for helping figure out who should be contacted now and what should be done.

That is normal part of work after any reorganization.

You assume too much.

I have seen it many times that when Bill leaves, the thing he was responsible for doesn't get picked up by anyone.

It doesn't necessarily even mean that the organization is "abnormal". Perhaps the reason Bill was let go was because X was not considered business-critical any more.