How to ACTUALLY avoid Steam Updating your game:
Many seem to know the recommendation:
just launch through SKSE and disable Automatic Updates in Steam
This is NOT a permanent solution!
The Permanent Solution:
- In Steam, right-click on Skyrim SE and select properties
- In the New Window, select Installed Files on the left
- In the Top right of the Window click on Browse
- Now File Explorer should be open move up two folders, you should be in a folder called "steamapps" now
- Look for a file called "appmanifest_489830" right-click on that file and select properties
- In the New Window, at the bottom above the OK button there should be Attributes: and two check boxes, checkmark the box next to read-only.
- DONE!
Steam will now be unable to update Skyrim SE ever again, until you go back to that file and remove the checkmark in the read only check box.