ISSUE

Saves
One minute your game is working fine. The next you go to play and the Load Game button has disappeared, there seems to be no way to play an existing save.

CC/Mods
Newly added CC/Mods do not show up in your game despite being properly installed (for help with installing 3rd party content see here) or CC randomly starts disappearing from your game.

Library items
Saving Lots, Rooms and Households to your in-game Library doesn't work

CAUSE

Most likely the internal storage on your Mac is too full and iCloud is pushing any newly added files to your Documents folder in the cloud. This can happen at anytime. You can check the storage level by clicking on the Apple in the top left corner of the menu bar then About This Mac, then the Storage tab.

By default, the game stores your Saves, Mods, Tray (where your Library items are saved) and all your user game data in a folder called The Sims 4 in Documents > Electronic Arts. If there is no Sims 4 folder in this location the game will recreate a new one each time it is launched. If you have iCloud Drive enabled and set to store your Desktop & Documents folder in the cloud it may be moving your existing Sims 4 folder to the cloud, hence the need to create a new Sims 4 folder every time you launch the game. You can check the status of various files and folders, and what the little cloud icons mean, here.

FIX

Firstly, you MUST free up some space on your Mac otherwise doing this is pointless. Empty the Downloads folder, go through your data and see what you no longer need, empty the Bin. If you have large Photos, Music and TV libraries consider storing them on an external drive. Instructions here on how to do this for your Photos library, the principle is the same for the Music and TV apps.
Open System Preferences and then your iCloud settings. What you click on can vary by macOS, check Apple's help page here if you can't immediately find where to look.
This is where you can choose what gets stored in the cloud. We are only interested in two settings, iCloud Drive and Optimise Mac Storage
Click on Options... in the iCloud Drive setting and make sure Desktop & Documents Folders is unchecked. Scroll down to the bottom of this Apple help page to understand what happens to your data when you turn off the setting.
Back at the iCloud settings window, Uncheck Optimise Mac Storage below the list of apps using iCloud. This means that your Mac will stop pushing files to the cloud when the internal storage is getting full. For more info on this setting read this help page.
Once you have successfully stopped iCloud from moving the contents of your Documents folder to the cloud you need to move the Sims 4 folder back into the default location - Macintosh HD > Users > [User] > Documents > Electronic Arts

Apple explain here what the status of the little cloud icons mean.

If you're feeling brave you could consider creating a symlink and storing your Sims 4 folder on an external drive. Or maybe change your EA App Games Library location to an external Drive.