The feat was printed in its altered form in KoK: Player's Guide to the Sovereign Realms
A bit of googling turned up this post in a GiTP thread where the mystery is helpfully explained by user ZamielVanWeber:
Here is the tale: Irresistible Spell was printed in the WotC official Kingdoms of Kalamar Player's Handbook as is. There are 3 errata filed to that book, none of which mention any change. However, in the unofficial 3.5 update to the KoK Player's Handbook called Player's Guide to the Sovereign Lands irresistible spell was changed to +10 to the save DC.
And indeed, on digging up a copy of the Player's Guide to the Sovereign Lands, it features the altered version of Irresistible Spell. The short description in the feats table still states it allows the caster to "forbid saving throws" (as can be seen in the free preview of the book), but its descriptive entry in the full text only has it add +10 to the DC.
What makes it "unofficial"?
The "official" vs. "unofficial" distinction owes itself to Kenzer & Company's complicated legal history with Wizards of the Coast. During the 3/3.5e era, Kenzer had a licence to produce D&D material using the KoK campaign setting, branded with the WotC logo (stating "official licensed product"). Kenzer made a lot of these books, mostly in the 2001-2003 period; the 3.5 revision came out in the middle of 2003, and so the latest of those "official licensed products" indicate use of the 3.5 revision in their text.
The original KoK: Player's Guide, however, was published in 2002, before the 3.5 revision, and an updated version wasn't released during that period. KoK: Player's Guide to the Sovereign Lands came out much later in 2006, does not bear the official WotC licensed product brand, and in fact takes care to note that it does not contain any material reprinted from other D&D sources, unlike its predecessor. It seems that by the time Kenzer wanted to print the 3.5 update for the Player's Guide they had abandoned or terminated the licence for some reason, so Kenzer simply published material in such a way that they figured they were legally in the clear without using WotC's copyrights (Kenzer has a lot of form for this, having previously published unlicensed KoK supplements for AD&D and going on to publish material for 4e without conforming to 4e's GSL).
The takeaway is that the original version of the feat as printed in the KoK: Player's Guide is technically from an official Wizards of the Coast 3e product, albeit only via licensing and written/published by a third party. The 3.5e revision from KoK: Player's Guide to the Sovereign Lands is not, so for a table that cares about whether or not books have the WotC brand, the 3.5e version isn't official. It was, however, all written and published by the same company and the same people, who actually own the Kalamar setting.