I believe there are ways to export the forum and turn it into a static html site, which could be hosted permanently to keep the information archive intact and accessible.
Did they leave or is the forum federated? 
@strypey did step up, and some time ago there was also interest of other people. Let’s see where things go.
Another option is that W3C SocialCG takes over the custodianship role in addition to the activitypub.rocks website, if they are willing.