Supported for what? As a way to access the esxi host to make permanent changes? No
As a way to log into a host to run diagnostic scripts or make temporary changes in order to workaround a problem? Yes. It's called "Tech Support Mode" because it's for... tech support. I think that the KB article implies that any changes you make without vmware support involved are on your own head. I also think that gone are the days that running an unsupported guest OS and having an unrelated support call resulted in a "get rid of that guest then call us back" type response. From the kb article listed in the question comment(which to me says it all):
"Tech Support Mode is provided for the purposes of troubleshooting and remediation. VMware support may provide directions to use TSM in many forms including but not limited to telephone, E-mail, knowledge base articles, documentation, and remote desktop.
VMware recommends using the vSphere Client or any other VMware Administration Automation Product to perform routine ESXi host configuration tasks that do not involve a troubleshooting scenario. For scripting and automation, VMware recommends using the vCLI, PowerCLI, or the vSphere SDK instead of TSM.
TSM may be required to test and debug pre, post, or first boot scripts that are used in conjunction with the new ESXi 4.1 scripted installation feature. VMware recommends limiting testing and debugging these scripts to non-production environments prior to roll-out into production."