In Sierra, I've got:

1. /Users/artie/Library/Application Support/AppStore, which is empty.

2. /Users/artie/Library/Application Support/App Store/updatejournal.plist, which is apparently my "Last 30 Days" list.

Code:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>autoInstalledUpdates</key>
	<array>
		<dict>
			<key>__isSoftwareUpdate</key>
			<true/>
			<key>installDate</key>
			<date>2018-01-09T18:35:29Z</date>
			<key>productKey</key>
			<string>091-57952</string>
			<key>release-notes</key>
			<string>The Safari 11.0.2 update is recommended for all macOS Sierra users and contains bug fixes and security improvements. 


For detailed information on the security content of this update, please visit: https://support.apple.com/kb/HT201222.
</string>
			<key>title</key>
			<string>Safari</string>
			<key>version</key>
			<string>11.0.2</string>
		</dict>
	</array>
</dict>
</plist>

3. /Library/Receipts/InstallHistory.plist, which has got 2,842 lines, lists all of my downloads, and ends with the three most recent macOS updates.

Code:
<dict>
		<key>date</key>
		<date>2018-01-24T08:26:15Z</date>
		<key>displayName</key>
		<string>Safari</string>
		<key>displayVersion</key>
		<string>11.0.3</string>
		<key>packageIdentifiers</key>
		<array>
			<string>com.apple.pkg.Safari11.0.3Sierra</string>
		</array>
		<key>processName</key>
		<string>softwareupdated</string>
	</dict>
	<dict>
		<key>contentType</key>
		<string>critical-update</string>
		<key>date</key>
		<date>2018-01-24T08:26:15Z</date>
		<key>displayName</key>
		<string>Security Update 2018-001</string>
		<key>displayVersion</key>
		<string>10.12.6</string>
		<key>packageIdentifiers</key>
		<array>
			<string>com.apple.pkg.update.os.SecUpd2018-001Sierra.16G1212</string>
			<string>com.apple.pkg.FirmwareUpdate</string>
			<string>com.apple.update.fullbundleupdate.16G1212</string>
			<string>com.apple.pkg.EmbeddedOSFirmware</string>
		</array>
		<key>processName</key>
		<string>softwareupdated</string>
	</dict>
	<dict>
		<key>date</key>
		<date>2018-01-24T08:26:15Z</date>
		<key>displayName</key>
		<string>iTunes</string>
		<key>displayVersion</key>
		<string>12.7.3</string>
		<key>packageIdentifiers</key>
		<array>
			<string>com.apple.pkg.iTunesX.12.7.3.delta</string>
			<string>com.apple.pkg.iTunesAccess</string>
			<string>com.apple.pkg.CoreFP</string>
			<string>com.apple.pkg.CoreADI</string>
			<string>com.apple.pkg.MobileDevice</string>
		</array>
		<key>processName</key>
		<string>softwareupdated</string>
	</dict>
</array>
</plist>

4. /Library/Application Support/App Store/adoption.plist.

Code:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>com.apple.garageband10</key>
	<dict>
		<key>apple-id</key>
		<string>[email address]</string>
		<key>dsid</key>
		<integer>204629426</integer>
	</dict>
	<key>com.apple.iMovieApp</key>
	<dict>
		<key>apple-id</key>
		<string>[email address]t</string>
		<key>dsid</key>
		<integer>204629426</integer>
	</dict>
	<key>com.apple.iWork.Keynote</key>
	<dict>
		<key>apple-id</key>
		<string>[email address]</string>
		<key>dsid</key>
		<integer>204629426</integer>
	</dict>
	<key>com.apple.iWork.Numbers</key>
	<dict>
		<key>apple-id</key>
		<string>[email address]</string>
		<key>dsid</key>
		<integer>204629426</integer>
	</dict>
	<key>com.apple.iWork.Pages</key>
	<dict>
		<key>apple-id</key>
		<string>[email address]</string>
		<key>dsid</key>
		<integer>204629426</integer>
	</dict>
	<key>eligibilityRead</key>
	<dict>
		<key>errorMessage</key>
		<string>ex.name (com.apple.jingle.foundation.exceptions.MZCodedException) -- ex.message (MZException code = MZFinance.AppAdoptionNotEligible, userInfo = null)</string>
		<key>errorMessageKey</key>
		<string>MZFinance.AppAdoptionNotEligible</string>
		<key>errorNumber</key>
		<integer>3552</integer>
		<key>status</key>
		<integer>-1</integer>
		<key>userPresentableErrorMessage</key>
		<string>Future updates of these apps can be found through Software Update.</string>
	</dict>
	<key>eligiblityReadDate</key>
	<date>2018-01-26T10:47:35Z</date>
</dict>
</plist>

Originally Posted By: artie
What I find odd is that [/Users/artie/Library/Application Support/App Store/updatejournal.plist...your ~/Library file, too] is user-centric despite the fact that it contains system-centric data.

I logged in to my test (Admin) user account, and found that while the App Store tells me that I'm up to date, it doesn't tell me what's been installed in the past 30 days, because, as alternaut guessed, it's dependent on the account that did the installations.

It seems totally counter-intuitive to me that only one user of a particular computer can determine what system-wide updates have been installed recently; that knowledge should be available to ALL users, Admin or not.

The new Great Equalizer is the SEND button.

In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire