What is the windows apppatch directory purpose and contents?

Solution 1:

The directory c:\windows\apppatch is used by the Microsoft Application Compatibility Toolkit

From How to use the Compatibility Administrator utility in Windows

You can use the Compatibility Administrator tool to quickly apply various program fixes (AppFixes, also known as "shims") to a program to determine their effectiveness. Compatibility Administrator reads the %SystemRoot%\AppPatch\Sysmain.sdb database file to produce a list of available fixes. When you select an AppFix, you can start the program executable (.exe) file, and the AppFix is applied. If a suitable AppFix is found, the tool eventually helps you generate and test matching file information.

So basically that directory contains details of application compatibility usable to seamlessly fix or patch applications that may be designed for older versions of Windows.