Not necessarily. Newer vSphere support older vDS versions (see e.g. https://docs.vmware.com/en/VMware-vSphere/7.0/com.vmware.vsphere.networking.doc/GUID-330A0689-574A-4...) so that you don't have to upgrade immediately, which - depening on your environment - may not even be possible, if you are still running mixed ESXi versions.
However, unless there are valid reasons for not upgrading, or in order to benefit from new features you may consider to upgrade.
André
