Those references are the problem here, not the targets they reference. We need to list those "entries in the archive table that reference the non-existing namesapce" and have them fixed or deleted.

If you make them show up in a different namespace, will there be an easy way of mass-deleting them with a bot? (we already have a bot or two with admin access, operated by one or two of our trusted admins). Or will they be lost in the namespace which they show up in? I guess we could designate a seldom-used namespace for them...

The issue has been resolved.

The Income Tax Ordinance requires a temporrary immediate solution while we continue to ponder the best permanent one.

In T298430#7608745, @Majavah wrote:

Hello!

delete the pages there

Just to confirm: deleting the namespace will make its pages (both currently existing and deleted) permanently unaccessible without sysadmin intervention, meaning that usual tools like Special:Undelete don't work with them. Is this fine with you?

Correction: Almost all pages are redirects

Thank you, Danny.

Link to paragraph in Village Pump.