This was changed for a bug fix: issue 11149. A workbook was being exported with worksheets which contained names longer than 31 characters. Since Microsoft Excel doesn't allow worksheet names longer than 31 character, they were getting truncated. However, in this particular case, the workbook already had another worksheet which had a name matching the truncated name of one of these other worksheets. This caused errors when the file was opened. Since Microsoft Excel doesn't allow more than 31 characters in the name anyway, we decided to throw an exception if a name was set with more than 31 characters.

This is not an issue since it's easy to limit the worksheet name but I get the exception when adding a worksheet with a long name to a workbook like this:

Workbook wb = new Workbook();
Worksheet ws = wb.Worksheets.Add("reallylongworksheetnamewillgivemeaniceexception");

I was just wondering this because long worksheet names had been working before and it seems to me that the limit was a new "feature".