The #!fold-case and #!no-fold-case directives are read as comments, which means that they are treated as whitespace (section 2.2). Unlike the other kinds of comments, their final delimiter is implicit. This means that (1#!no-fold-cases) reads as (1 s). This seems unfortunate.
We went with "followed by a delimiter". Formal syntax is not yet fixed: see #476.
WG1 voted that the delimiter for #!(no-)fold-case is whitespace.