we do not untag retired packages builds, the package gets blocked in koji which causes them to drop out of the compose tree.

Isn't that confusing, when looking build in Koji tagged f24 but not published in the repo?
I'd assume tag fN means "released at FedoraN GA".

Also for some packages "masher" did untag when retired e.g. openstack-nova was retired Dec 8:
{{{
Wed Dec 9 06:16:10 2015: openstack-nova-12.0.0-0.1.0rc1.fc24 untagged from f24 by masher
Wed Dec 9 06:16:10 2015: openstack-nova-12.0.0-0.2.0rc2.fc24 untagged from f24 by masher
Wed Dec 9 06:16:10 2015: openstack-nova-12.0.0-1.fc24 untagged from f24 by masher
Wed Dec 9 06:16:10 2015: openstack-nova-12.0.0-2.fc24 untagged from f24 by masher

}}}

Based on the above, that might be an issue or hiccup in masher scripts?

No it's operating as expected. If you look at the tags section (second table at the bottom) you can clearly see a minus against f24*/f25 to show it's blocked.

http://koji.fedoraproject.org/koji/packageinfo?packageID=12510

Sorry, openstack-nova was a bad example, let's focus on builds listed in description.
Those seem to have an issue b/c they were retired later but according to
https://fedoraproject.org/wiki/How_to_remove_a_package_at_end_of_life
it is still allowed to retire on Branched until the Final Freeze.

e.g. openstack-gnocchi http://koji.fedoraproject.org/koji/packageinfo?packageID=20367
is blocked in f25 but not in f24-Alpha and f24 is missing, and builds are tagged f24 unlike openstack-nova which is both blocked and untagged in f24.

List of packages retired in Feb 2016, which ended up in this limbo state is:

{{{
openstack-designate
openstack-gnocchi
openstack-heat
openstack-heat-gbp
openstack-heat-templates
openstack-manila
openstack-manila-ui
openstack-marconi
openstack-neutron-gbp
openstack-packstack
openstack-puppet-modules
openstack-sahara
openstack-swift
openstack-swift-plugin-swift3
openstack-tempest
openstack-tripleo
openstack-tripleo-heat-templates
openstack-tripleo-image-elements
openstack-tuskar-ui
openstack-utils
openstack-zaqar
}}}

Replying to [comment:1 ausil]:

we do not untag retired packages builds, the package gets blocked in koji which causes them to drop out of the compose tree.

I checked this again. At one point we started to untag builds for retired packages because it was causing problems at least for EPEL:
https://pagure.io/releng/blob/master/f/scripts/block_retired.py#_212
https://fedorahosted.org/koji/ticket/299

We untag epel packages only so that if they are added to RHEL we do not get the old epel package in the buildroot. EPEL uses a different process for its reasons to Fedora