I think they're mostly pretty obvious -- none of the following will ever be updated.
Just off the top of my head:
Commands:
BREAK
DRAWBOX
DRAWLINE
DRAWVLINE
KEYBD
KEYS
LIST
LOADBTM
MKLNK
PLAYAVI
PLAYSOUND
REXEC
RSHELL
SCREEN
SCRPUT
TEXT
VERIFY
VSCRPUT
Functions:
@CRC32
@DEC
@INC
@MD5
@SFN
Variables (not including the 32-bit-only variables):
CMDEXTVERSION
_CPUUSAGE
_MKLNK_ERRORS
_MKLNK_LINKS
_RUBYTYPE
_RUBYVALUE
_VIRTUALPC
And, of course, any version of Take Command / TCC that runs on Windows 32-bit. (Version 21 will be x64 only.)
This doesn't include the commands that are still commonly used but actually call other commands (like REN and TITLE).