Basically everything on FreeBSD supports it via libxo.

I don't have a list, but the modern replacement for "ifconfig" does JSON: "ip"

As does "lldpctl"

Ansible provides details about systems in JSON called 'facts'. The intention is to use these to inform automation

lsblk accepts a --json flag and can give you a lot of information (try lsblk --json --output-all). Very useful if your script needs to check what disks and partitions there are in the system.

TShark (the CLI companion to Wireshark) does with the `-T json` flag.

Probably not what you had in mind but, AWS CLI.

kubectl with “-o json”