Spec: String literals:

The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in rune literals (except that \' is illegal and \" is legal), with the same restrictions. The three-digit octal (\nnn) and two-digit hexadecimal (\xnn) escapes represent individual bytes of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual characters.

So \0 is an illegal sequence, you have to use 3 octal digits:

s := "golang\000"

Or use hex code (2 hex digits):

s := "golang\x00"

Or a unicode sequence (4 hex digits):

s := "golang\u0000"

Example:

s := "golang\000"
fmt.Println([]byte(s))
s = "golang\x00"
fmt.Println([]byte(s))
s = "golang\u0000"
fmt.Println([]byte(s))

Output: all end with a 0-code byte (try it on the Go Playground).

[103 111 108 97 110 103 0]
[103 111 108 97 110 103 0]
[103 111 108 97 110 103 0]