| I didn't expect brace expansion disable by default.
It isn't. You must have POSIXLY_CORRECT set if it was. And in
that case, if you're asking for a POSIX environment, it is reasonable,
I believe, that that is what you're given. Since brace expansion is
incompatible (currently) with POSIX requirements, it cannot be on by
default in POSIX mode.
If you don't care about being strictly POSIX compatible, don't define
POSIXLY_CORRECT.
| Reading again the man page (just 2886 lines...), it is clear
| that posix can be on by default,
It can (or so I believe, building ksh isn't one of my fetishes) but
no-one (and certainly not here) builds it that way.