I want to prevent non-users from seeing my posts in the same manner that FA has a non-user prevention option. (ie. "you must log in to see this content.")
Is there a way to do that here on e621?

Nope, no such feature exists here. All non-cub content is viewable without an account. There might be a way to prevent your content on here from being Google-able? At least, I think there used to be an obscure feature like that, IIRC.

EDIT: After some searching the forums, I can't find any proof this secret feature ever existed though...

I don't see the purpose given how easy it is to sign up. A user needs to be 18+ to come here even without an account, so an FA-like block on mature and adult posts for non-logged-in users won't have the same purpose.

watsit said:
A user needs to be 18+ to come here even without an account

by the terms yes, but in practice anyone can still click through the agreement and lie.

pantsu_smiley said:
I want to prevent non-users from seeing my posts in the same manner that FA has a non-user prevention option. (ie. "you must log in to see this content.")
Is there a way to do that here on e621?

That's just not how Boorus work. The only way you're ever going to stop a non-user from seeing it would be to send a takedown and stop posting. At that point though, all your content is likely still viewable over existing archives exposed as hidden Tor services or FurArchiver.

I think I saw on the cheatsheet that you can search for posts that are hidded to non-users, but I don't think that feature, if it has ever existed, was ever used.

watsit said:
I don't see the purpose given how easy it is to sign up. A user needs to be 18+ to come here even without an account, so an FA-like block on mature and adult posts for non-logged-in users won't have the same purpose.

https://www.theonion.com/i-am-under-18-button-clicked-for-first-time-in-history-1819570266