Here are build(s) of Csound that are odd, rare or not part of the official Csound release output. These are ordered by descending build date.

Windows XP

These releases are built for Windows XP (both 32 and 64-bit), but may be compatible with later versions of Windows. They have generally only been consistently tested on Windows XP x64 Edition and regular 32-bit XP.

Cabbage 2.7.0; 2021-06-18

Built on 2021-09-25
Only the 32-bit version is available. Notes

Csound 6.16.1; 2021-06-30

Built on 2021-07-14 (updated 2021-09-30 with CS_FLOAT_ALIGN fix)
Notes as with the 6.17 beta below.

Csound 6.17 beta (commit d606868); 2021-06-30

Built on 2021-07-10 (updated 2021-09-30 with CS_FLOAT_ALIGN fix)
Most things are supported and built, some aren't as detailed below.

Zipped binaries: Notes

DOS

Csound 4.11 (~2000)

The last known binary release to work with DOS, taken from the Csound book and compiled around 2000.
Csound 4.11 for DOS