nobody outside of your instance will see anything you post there

So you are saying that if let’s say a user from lemmy.world makes a post to the post-shutdown lemm.ee community, only users within lemmy.world can see the content? Did I understand that correctly? Basically each instance has a "copy of lemm.ee's community, and lemm.ee` is acting as the main distributor of events between these “isolated communities”.

Is there any indication for the lemmy.world people to notice that the instance hosting the community is gone? Wouldn’t there be cases where users continue posting to a community who’s instances have shutdown, without realizing that they are not visible outside?

every Lemmy instance stores copies of new data

What about the old data? Are they also stored retroactively once the link happens?

And about the linking, from what I gather, (taking your example) as long as any one user from lemmy.world from retrieves posts from lemm.ee, or if they subscribe to it, the “link” will happen. Is that correct?

Is there anyway to verify that the data is persisted on lemmy.world? As long as the post/community is visible right now from lemmy.world, would it be indication that it will be there even after lemm.ee shuts down?

Basically, I am mainly concerned whether my previous content on lemm.ee will be persisted.