When an old branch goes out of support, there are some things to garbage-collect.
(new codebase) Edit /var/portbuild/conf/server.conf with the following changes:
Delete new-branch from SRC_BRANCHES.
Delete SRC_BRANCH_old-branch_TAG =whatever
(both): umount a/snap/src-old-branch/src; umount a/snap/src-old-branch; zfs destroy -r a/snap/src-old-branch