add-column-nullable        safe; backfill in a follow-up
add-column-not-null        unsafe; require explicit DEFAULT or 2-step backfill story
drop-column                unsafe; require deprecation window first
rename-column-or-table     unsafe; require shim/dual-write window
add-unique-index           unsafe without prior dedup verification
type-change                unsafe; route via column-add + dual-write + drop
truncate / delete-from     unsafe; require Decision Packet
explicit-table-lock        unsafe in hot tables; verify maintenance window