Solution to the impending update: save your modded skyrim!

go into your steam folder→steamapps

And find the file named "appmanifest\_489830.acf"

Right click it, click properties, then tick the box marked read-only

Now steam no longer has windows' permission to update the game. (Idk how to do the same on linux, sorry)

The drawback if this is you'll no longer be able to launch skyrim through the steam launcher, you'll need to launch it via skse.exe or MO2

The other possible drawback (?) is when the update comes the creation store will stop working for you. (As it's designed for the current version of skyrim) But whatevs... @#$& paid mods anyhow.

Edit: ALTERNATIVE METHOD (that steam or windows can't circumvent somehow): Mod Organizer 2 has a "stock game" feature where you can have a separate install of your game that will never be updated:

In explorer go to your MO2 install folder and create a folder called "stock game" if it does not exist.

Open another window or tab and go to your skyrim se/ae install directory, copy everything inside

And paste it in stock game.

Redirect MO2 and the other apps to use these exectuables instead of the steam ones.

Delete gpu.txt from stock game if it exists and got copied. It's not needed

Done! Now you have a skyrim copy that will never update.

NOTE: **copy** the files, leave the steam originals intact