'Directly connected' to a project might mean a user of - but I assume you mean a major contributor to (as even small time users of free software often contribute something - bug reports, feature requests, code contributions, money, etc).

The page: https://pulsar.apache.org/powered-by/ suggests there's quite some number of corporate users who are happy to confirm they use this suite. I don't know how many of those you've heard of, though.

I suspect many private & government agencies around the world would decline to formally attach their name to any list like this, lest it be (mis)interpreted as an endorsement.