Sims 4 now saves in Onedrive only?

Could you right click on your documents folder and check if it's still local ? My guess is that it somehow got replaced with the the onedrive

Choose Properties - Path

Also what's your operating system ?

My laptop's Windows 10 and yeah my documents folder is still local but the game has created another documents folder and electronic arts folder within the onedrive folder which the game now saves to instead of the local one. I'm wondering if the only way to stop it is if I unlink my onedrive account and re-link it? Or untick the electronic arts folder in the onedrive settings?

Try what unticking does.

I have no experience with onedrive myself. Don't use it.

Do backup all your files to a place on your hard drive first.

I cannot overstate how inaccurate this statement is as of January 2019.

Instead of making my game able to play without syncing, it moved ALL 12,000 OF MY SIMS FILES to an inaccessible OneDrive folder away from My Documents.

DO NOT TAKE THIS ADVICE AS OF JANUARY 2019.

I am now sitting here trying to move the files back with an estimated timer of 19 hours.  Thanks!  ☹️

It seems that you by accident changed the default from saving locally to saving on Onedrive. As well as moving the files back make sure you follow the instructions on how to again make saving locally the default. And if you really have 12000 Mods/CC you might want to bring that down to something a little more manageable.