While new users won't need bot access immediately, there are two ways in which the autoconfirmed requirement can bite established users as well:

• if they never visited meta (e.g. an enwiki-only botmaster wants to switch to OAuth authentication)
• if they want to use a separate bot account

There isn't any point in trying to limit owner-only accounts (not after T142276 gets fixed, at least) so maybe just drop the mwoauthproposeconsumer requirement for owner-only?