When an old branch goes out of support, there are some things to garbage-collect.
Edit /var/portbuild/conf/server.conf with the following changes:
Delete old-branch from SRC_BRANCHES.
Delete SRC_BRANCH_old-branch_SUBDIR= whatever
umount a/snap/src-old-branch/src; umount a/snap/src-old-branch; zfs destroy -r a/snap/src-old-branch
You will probably find that the following files and symlinks in /var/portbuild/errorlogs/ can be removed:
Files named *-old_branch-failure.html
Files named buildlogs_*-old_branch-*-logs.txt
Symlinks named *-old_branch-previous*
Symlinks named *-old_branch-latest*