It depends on the game and what the mod maker decided.
I made the mod for the Divinity: Original Sin that changes a few bytes in the game XML files to allow for 4 players in game instead of just 2, since it was mostly supported but probably removed for console porting simplicity. This is a braindead simple mod that just needs to find some XML tag inside the embedded EXE/DLL file and update it. I didn't even have to update any checksums, etc.
When I published the mod I chose to target the hashes/offsets of the Steam EXE since that was what everyone had. So, while I didn't target the workshop (as this modification could not be done with it) I did target Steam end-users.
I made the mod for the Divinity: Original Sin that changes a few bytes in the game XML files to allow for 4 players in game instead of just 2, since it was mostly supported but probably removed for console porting simplicity. This is a braindead simple mod that just needs to find some XML tag inside the embedded EXE/DLL file and update it. I didn't even have to update any checksums, etc.
When I published the mod I chose to target the hashes/offsets of the Steam EXE since that was what everyone had. So, while I didn't target the workshop (as this modification could not be done with it) I did target Steam end-users.