Today, several in my mastodon timeline mentioned they finished the move. But without them mentioning, I, an outstander (i'm on another instance) wouldn't notice it.
What will happen, though, is that a portion of the users won't migrate. Either because they forget, or they can't be bothered, are "zombie accounts", or because its too challenging: it does involve down- and uploading and/or copypasting zips/datafiles. This means a bit of pruning or culling, and that could be considered good, IDK.
What will also happen, on a more technical level, is that other instances and maybe bots and automation will hit timeouts and connection errors when it really shuts down. Most instances and fediverse software can handle this just fine, it's built with this mind; it might at most cause some overhead and load. Some flakey or poorly developed software might crash or break (for a moment).