bug#56641: Deprecate `lsh`

[Stefan Kangas]

Mattias Engdegård <mattiase@acm.org> writes:

> Adding a few words to the doc string may suffice, but a byte-compiler
> warning would help.

What we have done in such cases is declare it obsolete but add a comment
that we want to keep it around for longer.  See `interactive-p', that
was not deleted in Emacs 28 despite having been obsolete for over a
decade.