/admin/config/registration is one switch and one message.
The switch
Accept registrations. On means /register works. Off means anyone visiting it is
sent to a closed page.
The switch is checked twice: once when the page loads, and again on the server when a form is submitted, inside the same transaction that creates the record. So closing registration cannot be beaten by someone who had the form already open.
The closed message
Free text shown on the closed page. Write something useful. "Registrations are closed" tells a delegate nothing; "Registrations closed on 14 March. Allotments go out by 20 March. Contact secretariat@... with questions" answers the three questions they were going to ask.
If registration reopens while someone is sitting on the closed page, they are sent back to the form.
Before you open
Everything in launching a conference should be done. In particular: committees exist and are active, the matrix has portfolios, every fee tier is defined, and an admin has verified the payment provider end to end.
Closing early
Closing is the first move in any incident. If bad registrations are arriving, or the matrix is wrong, or payments are misrouted, close intake before you start fixing. It stops the problem growing while you work. See when something goes wrong.
Mode overrides this
In Society mode registration is structurally off regardless of this switch, because there is no event to register for. See operating modes.