No extraterrestrial iridium anomaly: http://adsabs.harvard.edu/full/1989LPSC...19..733F
I'm not sure I fully agree with them, because there is quite a lot of Ir there and their threshold of 200 pg/g seems somewhat arbitrary to me. They claim the Ir was derived from local rocks, but that's a hell lot of iridium!
There is an an iridium anomaly. Read all about it here (paywalled) and here (open access).
but the KT boundary is the only one commonly mentioned…why?
Because it's the first one found, a nice story of serendipity, because it's the last really big one, and the one that killed the dinosaurs. Everything you need to make a good story - even though it's not unique.
Iridium (and other PGE) anomalies and pretty much common feature of impacts. This paper shows attempts to link mass extinction events with impacts. This is a controversial issue, because you can also link massive volcanic events with mass extinctions, not only impact. But that's not for now - they do show iridium anomalies for all impacts. That's another paper showing iridium anomalies for impacts, much older this time.