It was running M-Link from Isode: https://www.isode.com/ . Their focus and expertise is mostly military and government deployments, rather than public IM services. We are extremely grateful that they offered to sponsor the service with both M-Link and hardware back when we really needed it, and it has been rock solid for many years.
But we're looking forward to the flexibility we'll have with Prosody to make some changes to the service and add some features we've been missing that are understandably not on Isode's roadmap. For example, we've had registration closed for many years, but we're considering experimenting with invitation-based registration: https://blog.prosody.im/great-invitations/
