I forget all the details, but there are a variety of issues with readers that don't support them (and most don't):
- Displays soft hyphens as question marks throughout the text
- Displays soft hyphens as hyphens throughout the text
- Breaks on soft hyphens as expected, but doesn't display hyphens anywhere, including the actual line/word breaks.
Example with ADE here:
https://www.mobileread.com/forums/showthread.php?t=28108
And a laundry list of reading software here:
https://www.mobileread.com/forums/showthread.php?t=28139
I'm not sure if Calibre always deletes them, it may be dependent on which conversion features you have enabled - I think if you enable heuristics they get removed, but otherwise may be kept. See here:
https://www.mobileread.com/forums/showthread.php?t=57086
Another side effect is the soft hyphens will break text search in most (all?) ePub reading systems.
Further reading here:
http://infogridpacific.typepad.com/u...continued.html