You used to see it in workstation builds, although I can't remember the last time I saw that configuration lol. It's certainly not something you'd see often even then.
The workstation market is split between platforms using RDIMMs and UDIMMs (and has been for a very long time), but the RDIMMs still always have ECC and the UDIMMs for systems branded as workstations almost always have ECC.
My threadripper pro workstation (Lenovo p620) uses RDIMMs. So at least one manufacturer still has them in production.