after updating what is this new sign?

It will appear when downloading a Software update or downloading a file from Safari. Are you doing any of those actions?

I have not seen it reported yet, but I would also check if the phone is performing an iCloud Backup or if photos are currently syncing to iCloud.

• And did you verify if your device is backing up to iCloud at Settings > [name on top] > iCloud > iCloud Backup?
• If using iCloud Photos, check syncing status here: Settings > [your name] > iCloud > Photos

Apple has not updated their Status Bar Icons page yet, so it has not been included there with an explanation. Users have just been reporting what they have seen that will bring up that symbol.

The icon indicates that your iPad is synchronising with "something". Yours is one of many reports seen in recent days. At present, pending confirmation by Apple, this icon is believed to have some relationship to post-update synchronisation tasks performed by your iPad after updating to iPadOS 18.x.

Apple's documentation is often slow to catch-up with new features. Here is another status icon - that appears briefly when connecting some USB-C hubs - that Apple have yet to add to their documentation: