If the instance I started my community on shuts down, then the whole community is gone. Is there anything I can do as a mod to prepare for this so I can transfer everything onto a new instance? Or is everything lost if my instance shuts down?
The best you could do is try to archive the updates on an instance you control, but that is going to require you running an instance, writing custom code, and possibly breaking any GDPR protections you might have by not cross-honoring deletions.
There is a reason why a lot of Reddit subs who want to make their own Lemmy community create their own instances.
Setup your own instance (don't even need to allow other members), create the community there. Then just create some alt accounts on other instances that you think would be interested in your community. Subscribe to your community from those instances to at least get your stuff to appear. Than hope for the best.
Can't answer it but isn't the whole thing backed up (synced) to other instances? If I'm browsing lemmy.world community on my lemm.ee account, I'm actually only fetching data from lemm.ee. Only downside is that if there is an instance started after the community got lost, it will not have that community synced.
Do federated instances keep everything forever from communities someone is subscribed to? Or do they just keep a temporary cache that they can drop after no one has accessed it in a while?