Is there any way for a tenant admin to reclaim a registered application if the individual who registered it has left the company? I understand that more than one person should be designated as the owner. And there are a number of work-arounds available.
If the AD/Azure AD account has simply been disabled, an admin can re-enable the account, change the password, and log in under those credentials. Since the app continues to function when the owners account has been disabled, I foresee instances where the account has been deleted and its tombstone aged out.
Register a new application under another user's ID and update the project with this new ID/secret (although this requires figuring out what the proper app settings should be).
It would be nice if a quick/easy option were available for someone to reassign ownership of orphaned applications (and view a list of applications registered in their tenant).