I did a search from shitjustworks for “reddit die” and did not find https://lemmy.world/c/watchredditdie so I made https://sh.itjust.works/c/watchredditdie (unnecessarily). This should really not happen. When someone makes a community there should be a “ping” sent out to notify all other federated instances.
And from what I know, if I post to !sh.itjust.works/c/watchredditdie only users on sh.itjust.works will see the posts until other people from other instances randomly come across it somehow and subscribe? This really needs to be improved.
As a middle-ground, I think it’s enough to only sync the community name and user count and maybe the description. More isn’t shown in the search anyway and those 3 data points shouldn’t take too much storage.
Syncing name solves the problem of communities not showing up. The problem with only being shown posts in a community someone on the instance has already subscribed to is more difficult, as you wrote.
Descriptions can be of arbitrary length. Date and time of the most recent activity might be more realistic and useful.