From 19th July, Zoom will require that all meetings have a Passcode or a Waiting Room enabled. By 19th July, If neither is enabled, Zoom will enable a Waiting Room for your meetings.
If you add a Passcode to an existing meeting, the calendar invites will need to be resent to include the Passcode. New meetings will have the Passcode embedded in the meeting link automatically.
Find more about the change from ZOOM Support.