As I remember it, it has never been possible to paralyze undead and constructs. I also checked DDO Wiki ("Paralyzing") and sure there was this remark:
"Certain creatures, such as the Undead and Constructs, cannot be paralyzed. "
However, since probably update 19 (new enhancements), I am able to paralyze both constructs and undead with the paralyzing arrows on my arcane archer.
I went to test this out to make sure I wasnt imagining things and indead, I was able to paralyze zombies (drow zombies in Kings Forest, and zombies in Graverobber) and even wraiths (in Graverobber), and also Iron Golems (in Made to Order).
So I went back with a paralyzing longsword and tried the same. This time, nor the Golems, nor the Undead got paralyzed. Instead the floating damage said "Immune" for some of the damage types.
I search the forems for any comments on this, but could not find a single post. Am I the first to notice this? Or did I miss something that makes AA paralyzing different and that paralyzing undead and constructs this way is a normal thing?