The GCC 10 packages were also flagged for removal on upgrade to #OracleSolaris 11.4.66 (we already ship GCC versions 11, 12, & 13 to replace them).
Users of the gccgo compiler need to note that the removal of GCC 10 packages will also remove the libgo.so.16 library used by Go programs compiled with GCC 10, and that they thus will need to recompile any such programs with a newer version of gccgo before upgrading to SRU 66.