I bought "MYST Masterpiece Edition" a few decades back. When I just downloaded it again recently, installed it, and tried to run it it I got this error. "scummvm.exe - Entry Point Not Found - The procedure entry point IntializeConditionVariable could not be located in the dynamic link library KERNEL32.dll.". This game will no longer run on Windows XP, or any 32-bit version of Windows.:( It used to. But, it doesn't anymore now.

Does anyone know how I can download an older version?

One that will work with Windows XP for me now?

You could try to replace the scummvm version that comes with and get installed with the GOG installer, with the most recent release (2.5.0) which has versions for Windows XP (installer or zipped folder), 32bit and 64bit, as well as builds for newer versions of Windows 32bit:
https://www.scummvm.org/downloads/#release

Or try the development (daily) builds of ScummVM where reported issues since the release are potentially fixed:
https://buildbot.scummvm.org/#/dailybuilds (column "Branch Master").

From the daily builds page, if the Windows (32 bits) build does not work, the Windows 9x could work on Windows XP 32bit, but I'm not 100% sure.

The related bug report we had for this issue, appears to have been about 3 years ago, and the issue is considered to have been fixed as per the comments and the status of the ticker:
https://bugs.scummvm.org/ticket/10864