K-9 Mail never had this feature. Automatic setup didn’t work for your users in old versions of K-9 Mail. Now that we switched to using (a reduced version of) Autoconfig, it apparently still doesn’t work. I don’t see how that’s “more headache to end-users”. The situation for your users hasn’t changed at all.
One could argue that you’re making your users’ lives hard by not allowing the email address to be used as the username. Not every email client supports Autoconfig (including sending the email address). So some of your users always have the problem that they need to manually find the username to their email address.
But if you can automatically do the mapping during Autoconfig, you could also automatically do it before authentication. I realize this would require changes to your infrastructure. But it would benefit all users not using an Autoconfig client that sends the email address.
We’d then have to always ask for consent. That’s especially frustrating for users when after giving consent server settings can’t be found automatically because the provider doesn’t support Autoconfig.