The owner/admin of Masto Host has made various statements that he doesn't want to host more than a small target % of users and instances at any given time, because he wants to keep the fediverse diverse in hosts/doesn't ever want to be the single-point-of-failure in a federated network. (Which truly is a great attitude and one of the things I like about supporting masto.host.) So the impression I have here is that he is waiting for some of this massive influx of users to slough off a bit and give him a better idea of something resembling the new steady state before deciding on if masto.host's hosted percentage still fits within his self-imposed cap.

All of this also coincides with a massive migration to Mastodon v4 adding to current maintenance burdens.

So the short answer is: masto.host probably will open its doors again if it doesn't seem to be hosting too much of the Fediverse, and it will probably be at least a few more days if not a few more weeks before registrations open back up if they do.

(There are some alternative hosts to consider if you don't want to wait on masto.host.)