The original implementation for legacy layout in bug 135349 had the issue that it didn't take in account text-align-last: justify; (if text-align is different than justify).
IFC does not implement text-justify: none.
So I removed the existing legacy layout implementation of text-justify: none in https://github.com/WebKit/WebKit/pull/1308 to avoid confusion.
I think it should be easy to add support for both modern and legacy layouts, however it's probably worth refactoring to avoid duplicating the logic.
One testcase that would be nice to add is:
Currently, I think legacy layout handles this incorrectly, as the last line will still be justified.