It's widely implemented but not adopted. Most vendors implemented v4 to tick a box for some RFQ. I don't think any of the commercial fileserver vendors publish numbers but I used to work in that space and internally customer adoption of v4 was always a single digit percent. When I was on the customer side in HPC none of the supercomputer clusters were using v4 (I'm sure there are exceptions that I wasn't aware of).

I use it all the time. It's a huge improvement over NFSv3 for many reasons (performance, network ports, locking, ACLs, ...).

All the operating systems I names have fully implemented and mature NFSv4 stacks. Definitely not a MVP to tick a box.