Unable to unflag over 4000 flagged emails in Mail on iPhone

Just to confirm, the OP is talking about removing the flag from emails in a particular mailbox, such as Gmail.

I went into Gmail, selected all and then tapped the flag bottom left… this says move to junk, mark as read of flag… there is no unflag option.

We don’t need to remove the flagged virtual mailbox, but simply unflag a real mailbox full of flagged emails.

Also saying some have connected the phone to the Mac and used that to unflag, exactly how did they connect the phone to the Mac ?

Re: "Unflagging mail:

accidentally flagged all mail, now sitting on over 4000 flagged mail. Unable to unflag"

For iOS 26: Flag or block emails in Mail on iPhone - Apple Support

gives a few details. and how to unflag an opened email.

Eg: says you can:

"Create a Flagged mailbox

You can add a Flagged mailbox so all your flagged email messages are easily accessible in one location.

1. Go to the Mail app  on your iPhone.
2. Tap  in the upper-left corner until you see Mailboxes.
3. Tap Edit, then select Flagged."

A method you can try to unflag all:

Open the Maiil app, and tap on "Mailboxes",

then tap on "Flagged" to view all flagged emails

Next Tap "Edit"

Select all the flagged emails

Tap "Mark" (at the bottom)

Tap "Flag" > "Remove Flag"

__________________________

Else:

If you use iCloud: You can log-in to iCloud.com

Find and tap on the "Flagged" folder to view all emails.

Select all the flagged emails

Tap the "Flag" icon in the toolbar

Choose "Remove Flag" from the menu

Hope something here works for you : (Have also heard some have plugged iPhone into Mac and used Mac Mail to unflag, as selecting all the flagged emails is simpler there.) :-)