Highrez

I am using xmouse version 2.20.2 portable (later versions don't seem to have a "load profile" feature) on windows 10 and I think I'm using a "Willstar Wireless Cordless Mouse 2.4GHz Mice USB Dongle Optical Scroll For PC Laptop" from Walmart as that looks the closest to what I got, and I'm having a problem getting one of the buttons on the side of the mouse to emulate shift for the old "Diablo" game from 1996. I used to be able to hold the button down and the action of the shift key would happen in the game. I have the button set to the number 3 setting of during (press on down, release on up) and shift does activate outside of the game with xmouse, and the shift key does work in the game, but shift from the xmouse does not work in the game most of the time. occasionally it does work but very rarely. I also do block the original mouse input.
I don't remember if it happened after a windows update but I have the latest update of windows 10 (not 11). I have used xmouse for probably over a year. I use an English----- qwerty keyboard

If it works out of game, but not in game, then usually that means the game is blocking simulated input, but then if it works sometimes and not others, that makes less sense. It may be a timing issue, you could try adding {WAITMS:50} after the {SHIFT} and see if it makes ANY difference?

like:
{shift}{waitms:50}

Still with Method 3.

If that does not help, I'm not sure what the problem would be to make it works sometimes and not others.

Your right, it has to do with the game. I tried playing the game with different software (the game is for windows 95, I think) and it works with that software. I will discuss the issue with the people who make the software for me to play the game on windows 10.

Out of interest, what is the other software?
I'm surprised such and old game would have problems with simulated input (as in blocking the SendInput API so maybe there is some other problem I am not aware of).

The other software was what the creators called a "patch" but it didn't work like a normal game patch (something that changes features in the game) instead it allowed me to play the game in windows 10. The site I got it from is no longer there but it was called "patch 2.1.0"

Or did you mean the software that was blocking the input? The software that was preventing x-mouse from working was something called "devilutionx"