Issue Id isn't available via the GraphQL API

Unfortunately, you cannot get the id(gitlab internal id, iid) of an issue via the GraphQL Api, only via REST.
Is there any chance that this will change in the near future?

The Issue docs show an iid (Internal ID of the issue) field, so it looks it’s already available?

Unfortunately not, the iid ist just the the project internal id of an issue(example: #254)
An example for a gitlab internal id is gid://gitlab/Issue/27039960.

Ah, I got confused as you mentioned iid, which is the id within the project.
The global id is indeed not yet available.
I suggest you create a feature request asking to add this information in the future.