A problem I had with TeX years ago was that discardable items (glue items, kerns, and penalties) are not recoverable in the output routine. This would be quite useful for complex multi-column pages with insertions, especially if both columns and facing pages be required to balance perfectly.
My goal at the time was to create a much-too-long list, invoke the output routine, and then programmatically rearrange the contents to make a good-looking multi-column page, as in, say, an undergraduate textbook.
But discardable items really are discarded, and there’s no way to get them back.
Has LuaTeX (or any other TeX derivative) fixed this?