Are the strings in argv modifiable?

From the C11 standard draft N1570, §5.1.2.2.1/2:

The parameters argc and argv and the strings pointed to by the argv
array shall be modifiable by the program, and retain their last-stored
values between program startup and program termination.

They are modifiable. That means they are not string literals.

But be careful: the upper citation only refers to pointers to strings, excluding the obligatory null pointer at argv[argc]¹.
From the C11 standard draft N1570, §5.1.2.2.1/2 (same as above)¹:

argv[argc] shall be a null pointer

Notes:

• Something regarding this sentence:

  I know that you shouldn’t modify string literals as it can cause
  undefined behavior […]

  “can”? It does always. Undefined behavior includes expected, as if well-defined, and unexpected behavior.

_{¹ Thanks to @black!}