I'm talking about deletion of the application itself, not of revoking approval.

I see. Maybe you could just add these fields to json output to avoid troubles? I had a really bad time figuring out why my oauth lib is not compatible with MediaWiki :)

That's a developer's responsibility, not Mediawiki admins. If the developer wants to delete his app, there are no reasons to forbid him doing it. He could change his tokens to the new app, so the users won't notice anything.
There also could be "zombie" users, who used the app once and not returning back, so the outdated app will never be deleted.