In Oracle GI 18c, the OCR and Voting disks must be located on ASM. A certified NFS or 3rd party shared file system is not supported for these files (you can keep your database on NFS or 3rd party shared FS, but not the OCR or the voting disks). More information can be found in the documentation. Note that placing these files on an NFS or shared FS was deprecated in 12.2
In 19c, Oracle removed this restriction and OCR and Voting disks can be placed directly on NFS or shared FS again (read about it here).
You can read more about this (and more topics related to upgrades) in this post.