The guidelines proscribe declaring unused types, tags on structures or enums, macros, or labels.
Shouldn't there be a prohibition of undeclared variables, too?
-Stephen H. Westin
We are assuming that the question is intended to read "Shouldn't there be a prohibition of unused variables, too?"
Rule 2.7 covers variables that are declared as parameters to functions.
MISRA C:2012 permits other variables to be initialised and not used. However, if a variable is assigned a value, which is not used, then dead code will exist and rule 2.2 will be violated.
Unused initialisations are permitted because a common programming style is to use configuration files and conditional compilation. This can lead in some configurations to some variables being initialised but never used.