I eliminated all old config content and reinstalled
apt-get remove --purge postfix
apt-get install postfix

in /var/log/mail.log:
Feb 24 11:08:35 lap postfix[25109]: Postfix is running with backwards-compatible default settings
Feb 24 11:08:35 lap postfix[25109]: See http://www.postfix.org/COMPATIBILITY_README.html for details
Feb 24 11:08:35 lap postfix[25109]: To disable backwards compatibility use "postconf compatibility_level=2" and "postfix reload"
Feb 24 11:08:35 lap postfix/master[24994]: reload -- version 3.0.4, configuration /etc/postfix
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 12: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 40: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 41: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 44: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 45: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 46: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 47: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 48: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 49: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 50: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 53: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 54: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 56: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 57: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 58: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 59: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 62: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 63: using backwards-compatible default setting chroot=y
Feb 24 11:08:35 lap postfix/master[24994]: /etc/postfix/master.cf: line 64: using backwards-compatible default setting chroot=y

I totally agree that this is correct if a user had an old configuration.
I guess the default conf has changed, so I guess we should deliver a new conffile matching the postfix 3 config?
I didn't want to file a bug before asking how much of keeping it as-is was intentional