Thanks for your message and your interest in the SIPL code.
The code in the CERN SoC gitlab repository is free and open-source software that was developed by ATLAS L1CT, and for which we have not provided any license. It can freely be used for any CERN-related projects.
"SIPL" stands for "Serial Interface Protocol Lite" and is defined by PigeonPoint (nVent Schroff) in their BMR-A2F-ATCA Software Architecture. It is very much based on the "Intelligent Platform Management Interface Specification - Second Generation" in section 14 "IPMI Serial/Modem Interface", in particular, subsection 14.4 "Basic Mode" and 14.7 "Terminal Mode". PigeonPoint are adding a number of specific extension commands.
I do not think that, apart from those few additional commands, there is anything proprietary in the protocol, which essentially defines how to send and receive IPMI messages over a serial interface.
I am adding my colleagues Stefan and Markus in cc, who might want to comment.