When the server closes, the communities will stay visible on other servers, but any new content posted to them won't federate to other servers. As a result, they will live on in a sort of "undead" state. (Example: https://sh.itjust.works/c/bertstrips@feddit.de (feddit.de shut down about a year ago))
To address this, I would recommend the following actions:
Set each community to "only moderators can post"
Include a link to wherever the community has decided to move to in the sidebar of the community.
Prepend the string "[Retired]" or "[Closed]" to the display name of the community.
This would inform anyone who comes across the communities afterwards what the situation is and prevent them from inadvertently posting to a "dead community".
Thank you for making it! Clickable mention - !Bluesky@lemmy.zip