Correct. NT 3.1 ran on x86, Alpha and MIPS. NT 3.5 added PowerPC support. As you say, SPARC was never supported.

Windows 2000 had both x86 and Alpha support up through the release candidate, but Alpha was dropped before final release. I actually had an Alpha workstation running Windows 2000 (for fun) back in the mists of time.

There's also the Beta Whistler AXP64 Build which was used to port Windows over to 64-bit before there was any 64-bit hardware available that could run NT. The people in the virtuallyfun discord got that running (I am part of that! I have an AlphaServer 800 5/500 next to me, with some hacks to the setup and telling it what HAL DLL to load it installed perfectly!)

Later there was Windows XP 64-Bit Edition for Itanic Workstations and there was Windows Server up to 2008 R2 for Itanic Servers.

Itanium XP was weird, it's like that super early AMD64 version where more of the OS is 32bit than 64bit, with no high theming.

It was still exciting at the time.

IIRC it was even weirder, because it was a XP branded Server 2003 under the hood.

It's even weirder than that :) The initial release of XP for Itanium was more or less a stripped down XP build with limited 64 bit support. They then quickly gave up on that approach and released a second version of Windows XP for Itanium, together with the launch of Itanium 2 support, which was, as you said, a rebranded Server 2003.

One could say, that they tried really hard. ;)

There was an announcement that Intergraph would port Windows NT to SPARC: http://ftp.lanet.lv/ftp/sun-info/sunflash/1993/Jul/55.11-Sun...

Guess it never went anywhere.

It didn't. There are rumors it got out to a handful of alpha/beta sites, but it was never a supported product.