Index sorting commands work only in TeX backend?

[David Kastrup]

Apparently, stuff like @sortas and @txiindexbackslashignore work only
with the TeX backend.  All of

       For the printed output, you may specify an explicit sort key for an
    index entry using '@sortas' immediately following the index command.
    For example: '@findex @sortas{\} \ @r{(literal \ in @code{@@math})'
    sorts the index entry this produces under backslash.

       To reduce the quantity of sort keys you need to provide explicitly,
    you may choose to ignore certain characters in index entries for the
    purposes of sorting.  The characters that you can currently choose to
    ignore are '\', '-', '<' and '@', which are ignored by giving as an
    argument to the '@set' command, respectively, 'txiindexbackslashignore',
    'txiindexhyphenignore', 'txiindexlessthanignore' and
    'txiindexatsignignore'.  For example, specifying '@set
    txiindexbackslashignore' causes the '\mathopsup' entry in the index for
    this manual to be sorted as if it were 'mathopsup', so that it appears
    among the other entries beginning 'M'.

don't seem to have an effect on the index in the resulting Info file.
That's a bit lack-lustre and makes the feature considerably less useful.
While the Info file tends to be navigated with key sequences rather than
by looking explicitly in the index, the same does not hold true for
HTML, and the average user these days is more likely to be using HTML
than either Info (with its key navigation) or PDF (which is the only
backend apparently supporting index sorting criteria).

-- 
David Kastrup