If one created an application for testing purposes, he should have the right to delete it. Otherwise there will be a ton of "Hello world" applications for approval, which will cause delays.
There is also should be a button or checkbox "Send this application for approval" instead of just requesting approval on creation.
If one created an application for testing purposes, he should have the right to delete it.
What if others already used it? Should she only have the right to delete if there are zero other users?
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There also could be "zombie" users, who used the app once and not returning back, so the outdated app will never be deleted.
I think all the use cases for this are covered by T60937: Add "under development" stage before "proposed" stage for OAuth consumers and T65712: OAuth: Developers should be able to revoke their own tool's approval.
Declining in favor of T65712: OAuth: Developers should be able to revoke their own tool's approval due to lack of use cases. There is little difference between a deleted and a disabled application - mostly just how they appear in navigation/search, and that needs to be rethought anyway. (In the rare cases where something needs to be made invisible, suppression already exists. But there is no reason owners need to be able to do that.)