Do all STC IDs or UID that are accessing UNIX services need to have OMVS Segment ID and Group ID? Is it because the default ID is not being used already in new z/OS and ACF2 Releases?
An OMVS segment would be the ACF2 USER and GROUP profile records that define a UID and GID for an OMVS user.
If your site has ACF2 GSO UNIXOPTS DFTGROUP and DFTUSER defined then you would not need to define a USER or GROUP profile specifically for the STC.
You can check this by doing a TSO, ACF, SHOW STATE,
and check for OMVS DFT LID=xxxxxxxx and OMVS DFT GRP=xxxxxxxx under "OPTIONS IN EFFECT".