vhdl-highlight-forbidden-words

vhdl-highlight-forbidden-words is a variable defined in `vhdl-mode.el'.
Its value is

nil

Documentation:
Non-nil means highlight forbidden words.
The reserved words specified in option `vhdl-forbidden-words' or having the
syntax specified in option `vhdl-forbidden-syntax' are highlighted in a
warning color (face `vhdl-font-lock-reserved-words-face') to indicate not to
use them.

NOTE: Activate the new setting in a VHDL buffer by re-fontifying it (menu
entry "Fontify Buffer").

You can customize this variable.