Only within supercomputers (including the smaller GPU ones used to train AI). Normal data centers use Cisco or Juniper or similarly.well known Ethernet equipment, and they still do. The Mellanox/Nvidia Infiniband networks are specifically used for supercomputer-like clusters.
Mellanox Ethernet NIC got used a bunch of places due to better programmability.
You seem to have a narrow definition of “normal” for datacenters. Meta were using OCP mellanox NICs for common hardware platforms a decade ago and still are.
Mellanox IB were ubiquitous in storage networking. None of the storage systems I worked on would have been possible without mellanox tech.
Most people on the TrueNAS/FreeNAS forums use 10gb Mellanox nics that are sold on eBay. The sellers get them after server gear gets retired.