allout-substring-no-properties is an alias for `substring-no-properties' in `allout.el'.

(allout-substring-no-properties STRING &optional FROM TO)

Return a substring of STRING, without text properties.
It starts at index FROM and ends before TO.
TO may be nil or omitted; then the substring runs to the end of STRING.
If FROM is nil or omitted, the substring starts at the beginning of STRING.
If FROM or TO is negative, it counts from the end.

With one argument, just copy STRING without its properties.