Pretty much every major update affects mods. We dont know the extent because we dont have the dev kit in our hands at this time. Once we do, mod creators who are interested in maintaining their projects will get to work. Mods that are not updated for 3.0 will be unknown if they will work after the update. Any mods that hard edit base game bluepritns or other assets will most be affected by it (if you are running a heavy amount of mods, then you likely have several that do this).
Can you force steam to not update the game? Ya, but not forever. There are likely guides on the internet that will explain, but thats not my area. You will also need to manually backup all your mods and tell your game to use the older versions instead of any mods that are downloaded after they update (that one yoi cant prevent no matter what). Any mods that yodate for 3.0 will not be compatible with 2.8