org-ascii-underline

org-ascii-underline is a variable defined in `ox-ascii.el'.
Its value is

((ascii 61 126 45)
 (latin1 61 126 45)
 (utf-8 9552 9472 9548 9476 9480))

Documentation:
Characters for underlining headings in ASCII export.

Alist whose key is a symbol among `ascii', `latin1' and `utf-8'
and whose value is a list of characters.

For each supported charset, this variable associates a sequence
of underline characters. In a sequence, the characters will be
used in order for headlines level 1, 2, ... If no character is
available for a given level, the headline won't be underlined.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.4 of Emacs.