As of June 2021, Ubuntu Xenial stemcells are in Extended Security Maintenance (ESM). For more information, refer to the Ubuntu lifecycle and release cadence.
VMware has a commercial agreement with Canonical to continue providing secure and patched Xenial stemcells through April 2024, but only for Tanzu-related products.
VMware Tanzu only supports the stemcells available here: Stemcells (Ubuntu Xenial).
The Ubuntu Bionic stemcell is available here: Ubuntu Bionic (18.04).
Note: The Ubuntu Bionic stemcell is not supported by Tanzu-related products.