From what I can tell, this has nothing to do with their domain expiring / them forgetting to pay their domain bill. WHOIS records show it had been registered for many years, and domain registration is paid for in advance:
The domain status appears to have changed. The June 10th 2023 WHOIS data showed it as:
This is what you expect for a domain that is not currently being transferred.
The status now is:
Something is clearly going on with the domain, and it's not forgetting to pay a bill. All DNS records are gone, so this doesn't look like an oopsie there either.
I think all of these are possibilities:
Owners decided to pull the plug on it
This is a bungled attempt to transfer the domain to another registrar
Someone managed to break into their registrar account and is trying to transfer the domain away
I think these are unlikely:
Legal/law enforcement action
Accidentally deleting DNS records
It will be interesting to see how this develops. If vlemmy is truly gone, then this is a significant loss to the Fediverse, as they were the only larger Lemmy instance to have a no defederation policy.