openmediavault v7 uses a SaltStack as configuration management system. This means that making changes to services managed by OMV, such as NFS, should be done through its own tooling instead of directly modifying the configuration files of the target service.
The options of the NFS daemon in OMV can be set with the custom OMV_NFSD_MOUNTDOPTS variable. Once the change is done, it is necessary to stage and deploy the configuration with Salt to get that change translated into the actual configuration files of nfsd:
omv-env set -- OMV_NFSD_MOUNTDOPTS "--no-nfs-version 3"
omv-salt stage run prepare
omv-salt deploy run nfsIt must be noted that the command omv-env set overwrites the given variable. Hence, the previous example not only disables NFSv3 on the server side but also removes any other options passed by default to nfsd in OMV. For instance, the option --manage-gids will be removed and as a side-effect become disabled.