1. Are the unregistered participant records soft-deleted and still present in the database (or in a backup), and if so, is it technically possible to restore them, so the participants don't have to register again?

Yes, we can restore them. For future reference, the query would be:

UPDATE ce_participants SET cep_unregistered_at = NULL WHERE cep_event_id = 3804 AND cep_unregistered_at = '20260602150230';

I'll try to run that soon.

1. If there is currently no mechanism to restore accidentally removed participants, please consider this a request for such a provision (an "undo" / restore option for organizer-side participant removal), since accidental removal is easy and currently appears irreversible.

While I agree that ideally we wouldn't need to manually run a query in these cases, there's a decent amount of work involved in that, including considerations as to whether we should really let people see deleted participants. Also, and most importantly, in order to remove everyone you have to 1) select all participants 2) click a red button that says "remove" 3) confirm the action in a dialog with a big red button. It can happen, but I wouldn't necessarily agree with the statement that "accidental removal is easy".

UPDATE ce_participants SET cep_unregistered_at = NULL WHERE cep_event_id = 3804 AND cep_unregistered_at = '20260602150230';

Sounds good to me, I would just suggest to check the number of affected rows, just to be sure, like:

SELECT COUNT(*)
FROM ce_participants
WHERE cep_event_id = 3804
  AND cep_unregistered_at = '20260602150230';

In T427962#11978529, @cmelo wrote:

UPDATE ce_participants SET cep_unregistered_at = NULL WHERE cep_event_id = 3804 AND cep_unregistered_at = '20260602150230';

Sounds good to me, I would just suggest to check the number of affected rows, just to be sure, like:

Yup, I'd done this before and reported in IRC, that the query confirmed what is stated in the task description: 61 records affected.

Mentioned in SAL (#wikimedia-operations) [2026-06-02T18:21:39Z] <Daimona> Running query from T427962#11978299 in x1.wikishared