Some time ago, I’ve modified the tabbed rxvt-unicode. Upstream did not want it while people started modifying the extension even more and so tabbedex came to be. Its code can be browsed online or cloned from https://codeberg.org/mina86/urxvt-tabbedex.git.

tabbedex carries a lot of changes and it would be futile to try and list them all here especially as the list would likely become out of date soon. Because of that, I recommend taking a look at the source code, which has, at the very beginning, a comment with a list of changes made.