The latest Linux kernel will no longer include support for Intel/HP’s Itanium processors <https://www.theregister.com/2021/02/01/linux_pulls_itanium_support/>. Of course, prior Linux kernels up to 5.10 from December presumably still include such support, while the last version of HP’s own HP-UX 11i for its Itanium-based Integrity servers was v3 from 2007 <https://en.wikipedia.org/wiki/HP-UX#Release_history>. The last working version of Microsoft’s Windows Server for that platform, 2008 R2, came out just a couple of years later, and already dropped some features compared to editions for other platforms <https://en.wikipedia.org/wiki/Windows_Server_2008_R2#Editions>. Intel officially claims to be shipping new Itanium processors up to July this year, but it seems pretty clear the order book is empty. So it looks like the last OS still standing for that platform will be Linux.