Every major macOS upgrade leaves debris: the full installer in Applications, delta patches in Software Update's stash, and an APFS snapshot marking the pre-update state. Together they routinely hold 10-20 GB — all counted as System Data or Purgeable.

Safe to delete (after a week of stable running)

# the full installer app, if present
ls /Applications | grep -i "Install macOS"
sudo rm -rf "/Applications/Install macOS *.app"

# old Time Machine local snapshots (pre-update ones included)
tmutil listlocalsnapshots /
sudo tmutil thinlocalsnapshots / 999999999999 4

The installer app is re-downloadable any time; the pre-update snapshot only matters if you plan to roll back the entire OS within its retention window. If the upgrade has proven itself, both are dead weight. Details on snapshot thinning live in the snapshots guide.

Do NOT touch

• /System/Volumes/Update — managed by the OS mid-update; deleting mid-flight bricks updates
• macOS Install Data folders while an update is pending — finish or cancel the update first
• Anything under /System or /private (see the emergency cleanup order)

Prevent the pile-up

Install the upgrade, run it the same day, then immediately re-check Storage: the 48 hours after an upgrade is when its debris is fully claimable. Fold the check into the same quarterly pass as your LaunchAgents audit. Full background in System Data explained; to size your own pile first, the estimator takes ten seconds.