Table 12 on page 64 of https://www.intel.com/content/dam/support/us/en/documents/server-products/Intel_S2600ST_TPS.pdf, showsthe lowest supported data rate is defined as 100 Mbps, which shows it can be done. How?
"ADAPTER NOT SUPPORTED
This device cannot be configured by Intel PROSet Adapter Configuration Utility. Use the Intel PROSet Module for Windows Powershell interface or Windows Device Manager to configure the device. Consult the user guide for more information."
Windows Device Manager was not able to change port speed, FYI.