The content hosted on the site will be gone. with the mastodon stuff they already have a way to download your content and be able to reupload it on another instance (migrating). but that isn't a thing for lemmy/kbin just yet.
notably, beehaw just announced they're defederating from lemmy.world so go check that drama if you wanna see what happens.
If I post content, should I be basically spreading it across several instances to ensure my contributions aren't lost when someone else closes up shop?
Yeah if all your posts are on a single instance, then naturally when that instance closes that's going away lol. the nature of the fediverse is federation and decentralization so you should just post freely to various instances as you please.