Using soft hyphens is not recommended:

Okay, it's not a bug, but the U+00AD character is established in so many
other contexts, so not mentioning them in the spec is likely a result of
human error. I agree we cannot recommend them as portable solution now, but
I think one should at least recommend them in those cases where it works.

The XSLFO spec leaves a lot of details open, some
explicitely and intentionally, some probably quite
unintentionally. They'd never got it out of the door
otherwise. It is commonly understand that the Unicode
related standards should guide character handling,
which describe how soft hyphens are handled. It's
just that nobody yet got around to implement it in
FOP. It will be ultimately done though.