Here is a normal vacuum (without FULL) of pg_statistic. It does report a large number of dead-but-not-removable rows:
# vacuum analyze verbose pg_statistic;
WARNING: oldest xmin is far in the past
HINT: Close open transactions soon to avoid wraparound problems.
INFO: vacuuming "pg_catalog.pg_statistic"
INFO: index "pg_statistic_relid_att_inh_index" now contains 482186 row versions in 1325 pages
DETAIL: 0 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_statistic": found 0 removable, 567740 nonremovable row versions in 34667 out of 34667 pages
DETAIL: 558093 dead row versions cannot be removed yet.
There were 21663 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.12u sec elapsed 0.13 sec.
WARNING: oldest xmin is far in the past
HINT: Close open transactions soon to avoid wraparound problems.
INFO: vacuuming "pg_toast.pg_toast_2619"
INFO: index "pg_toast_2619_index" now contains 3319 row versions in 12 pages
DETAIL: 0 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
INFO: "pg_toast_2619": found 0 removable, 3319 nonremovable row versions in 667 out of 667 pages
DETAIL: 3097 dead row versions cannot be removed yet.
There were 0 unused item pointers.
0 pages are entirely empty.
CPU 0.00s/0.00u sec elapsed 0.00 sec.
VACUUM