Looking at the revision history from SCO's website, is that true SCO is the one who maintains the ABI standard? If not, who are the people maintaining the standard?
The Xinuos page hosting the System V generic ABI states that
The contents of these chapters is being actively maintained (in part) through an industry committee chaired by Intel. This group's efforts are greatly reduced from when they first were extending ELF to support 64-bit architectures, but there is still some active development.
(Xinuos was formed after the acquisition of OpenServer and UnixWare from the SCO Group.)
SCO is dead and has been dead for a long time, https://en.wikipedia.org/wiki/Santa_Cruz_Operation. The most recent edition of the Linux Standards Base (LSB) is at http://refspecs.linuxfoundation.org/lsb.shtml. Check the LSB Core specifications. Note that recently many Linux distributions have begun to distance themselves from LSB.
An ABI is very very architecture-dependent... and sometimes compiler-dependent (C++ is notorious for different compilers having different ABIs). You should find and read the specific documentation for the processor architecture of and possibly language of interest.