Based on revisions 1.7 and 1.11, I think this line should have been removed as part of revision 1.11.
Index: sys/arch/amd64/stand/libsa/cmd_i386.c =================================================================== RCS file: /cvs/src/sys/arch/amd64/stand/libsa/cmd_i386.c,v retrieving revision 1.13 diff -u -p -r1.13 cmd_i386.c --- sys/arch/amd64/stand/libsa/cmd_i386.c 15 Mar 2019 06:53:37 -0000 1.13 +++ sys/arch/amd64/stand/libsa/cmd_i386.c 6 May 2019 23:43:09 -0000 @@ -96,7 +96,6 @@ Xregs(void) int Xboot(void) { - printf("Not supported yet\n"); int dev, part, st; struct diskinfo *dip; char buf[DEV_BSIZE], *dest = (void *)BOOTBIOS_ADDR;