This more than likely affects only a small amount of Windows 11 users, but if you want Math Input Panel back in Windows 11 (it was removed), just follow these steps:
1. In a Windows 10 installation, navigate to X:\Program Files\Common Files\microsoft shared\ink (where X is the drive letter of Windows 10).
2. Look for mip.exe and copy it to the desktop. Next, go to the en-us folder inside of the ink folder and look for mip.exe.mui. Copy this to the desktop as well.
3. In the Windows 11 installation, copy both of those files to the same directories (so, X:\Program Files\Common Files\microsoft shared\ink and X:\Program Files\Common Files\microsoft shared\ink\en-us). Create a shortcut for Math Input Panel, which should auto place itself on the desktop. Open it up and voila, working Math Input Panel!