@7pckc4k8a5eh I take it your EA account already owned Sims 4? If not, if you were only accessing the content through Steam, of course you wouldn't be able to play once the accounts were unlinked. But I'm guessing you know that already.
Anyway, please contact EA customer support and ask that the Steam version of the Sims 4 base game be removed from your account. If a support rep says it's not possible, please let them know that it is and also Sims 4 is a free game so there are no consequences to removing it. If they need to remove your EA-owned base game too, that's fine, you can claim it again afterwards; just make sure they're not removing your owned packs.
I would strongly recommend using live chat rather than email. If you don't see the option, try this link:
https://help.ea.com/en/help-contact-us/?product=the-sims-4&topic=missing-game&category=manage-my-account&subcategory=find-missing-game
Make sure you're signed into an EA site (this one is fine) before clicking the link and that the location in the lower-right corner of the browser window is set to U.S. or U.K.
If you don't get anywhere with support, please post back and let me know.