As part of T385341, we are adding a config setting to let communities choose the namespaces where event registration can be used. We have event page inputs in Special:EnableEventRegistration, Special:EditEventRegistration, and Special:GenerateInvitationList. Previously, these would be restricted to the Event: namespace.

However, core's HTMLTitleTextField and the underlying TitleInputWidget only support restricting pages to a single namespace. Hence, we are unable to restrict the results when there are 2 or more allowed namespaces. For the MVP, we are handling this by removing the field's namespace restriction, and using custom validation instead. For the future though, we might consider updating the core widgets to accept multiple namespaces, and switch back to the built-in validation.