Disregarding our "catch all" owners (ENG_REVIEW_OWNERS in the root, thakis in third_party, API_OWNERS in blink), there are some pieces of the code without any active code owner at all. Here "active" is defined as "having done a review in the Chromium repo the last 14 months".

Using a script [1] and some performance optimizations in the owners system [2] and removal of the reviewers mentioned above I got this list of code without any active owner:

=== Files/dirs with 0 active (14 months) owners ===
	third_party/cld_3			''
	third_party/gestures			''
	third_party/libevdev			''
	third_party/pexpect			''
	third_party/rnnoise			''
	third_party/usrsctp			''
	third_party/wds			''


Furthermore there are some 30 pieces of code (also mostly in third_party) which has a single active owner and is at risk:

=== Files/dirs with 1 active (14 months) owners ===
	codereview.settings			'agable@chr'
	content/public/android/java/src/org/chromium/IsReadyToPayService.aidl			'palmer@chr'
	content/public/android/java/src/org/chromium/IsReadyToPayServiceCallback.aidl			'palmer@chr'
	content/public/android/java/src/org/chromium/payments_common.aidl			'palmer@chr'
	google_apis			'rogerta@chr'
	third_party/accessibility-audit			'aboxhall@chr'
	third_party/android_media			'mlamouri@chr'
	third_party/android_opengl			'dtrainor@chr'
	third_party/android_protobuf			'nyquist@chr'
	third_party/android_swipe_refresh			'tedchoc@chr'
	third_party/ashmem			'digit@chr'
	third_party/blanketjs			'jamiewalch@chr'
	third_party/blink/renderer/README.md			'haraken@chr'
	third_party/devscripts			'thestig@chr'
	third_party/espresso			'jbudorick@chr'
	third_party/fips181			'wfh@chr'
	third_party/gif_player			'tedchoc@chr'
	third_party/gvr-android-keyboard			'vollick@chr'
	third_party/iccjpeg			'fbarchard@chr'
	third_party/jsoncpp			'sbc@chr'
	third_party/junit			'jbudorick@chr'
	third_party/libpng			'scroggo@chr'
	third_party/markdown			'rockot@goo'
	third_party/mocha			'michaelpg@chr'
	third_party/mockito			'jbudorick@chr'
	third_party/motemplate			'rockot@goo'
	third_party/openh264			'hbos@chr'
	third_party/pymock			'sbc@chr'
	third_party/qunit			'jamiewalch@chr'
	third_party/requests			'jbudorick@chr'
	third_party/simplejson			'rockot@goo'
	third_party/sinonjs			'jamiewalch@chr'
	third_party/spirv-headers			'dsinclair@chr'
	third_party/usb_ids			'reillyg@chr'
	third_party/visualmetrics			'nyquist@chr'
	third_party/xstream			'yolandyan@chr'
	third_party/yasm			'dalecurtis@chr'


Full output including code with only 2 owners, and a list of inactive code owners owning much code can be found in elsewhere. [3]

I don't know if all of the seemingly inactive code owners actually are inactive. Maybe there just hasn't been anything for them to review the last 14 months. Though 14 months is a pretty long time. A shorter time grows the list a bit but not much.

[1] https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1406693/1/owner_stats.py
[2] https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1404172
[3] https://pastebin.com/F4bhwvSG