The implementation of this is HTMLFrameSetElement::anonymousNamedGetter. Based on that, here's a test to check for support:
https://software.hixie.ch/utilities/js/live-dom-viewer/saved/4928

It's supported in Chromium and WebKit, but not Gecko or EdgeHTML.

Given this, measuring how often this getter returns anything other than undefined (null internally) would be a good start. Luna?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/026ac85facd73a803f14af641d3f5ef26c2953b5

commit 026ac85facd73a803f14af641d3f5ef26c2953b5
Author: Shanmuga Pandi M <shanmuga.m@samsung.com>
Date: Mon Nov 20 10:22:21 2017

Measure HTMLFrameSetElement's non-null anonymous name getter

This patch measures how often getter returns anything
other than non null.

Bug:  695891 
Change-Id: I62ea96d6ccf9c0dea0804dcab221e71b55ffca37
Reviewed-on: https://chromium-review.googlesource.com/773520
Commit-Queue: Shanmuga Pandi <shanmuga.m@samsung.com>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517789}
[modify] https://crrev.com/026ac85facd73a803f14af641d3f5ef26c2953b5/third_party/WebKit/Source/core/html/HTMLFrameSetElement.cpp
[modify] https://crrev.com/026ac85facd73a803f14af641d3f5ef26c2953b5/third_party/WebKit/public/platform/web_feature.mojom
[modify] https://crrev.com/026ac85facd73a803f14af641d3f5ef26c2953b5/tools/metrics/histograms/enums.xml

This non-standard getter was added 13 years ago to fix a compatibility issue at the time: https://trac.webkit.org/changeset/8717/webkit

The commit above is present since M64, and according to https://www.chromestatus.com/metrics/feature/timeline/popularity/2235 usage never goes above 0.0001%.

Shanmuga, are you going to follow up and consider deprecating+removing this?

Marking as available. In httparchive, it looks like there aren't any pages that hit the HTMLFrameSetElementNonNullAnonymousNamedGetter counter during page load:
https://bigquery.cloud.google.com/savedquery/762219082167:df7f9e5098ad45808a3e398e4adb13c7

Given the low overall usage, and the fact that this is measuring the getter and we can't tell how the non-null return value is used, if at all, I would support deprecation and removal of this.

i will proceed to deprecate and remove this feature.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b7a3fd5d28f223b18d5d8660aa57ce190678b921

commit b7a3fd5d28f223b18d5d8660aa57ce190678b921
Author: Hwanseung Lee <hs1217.lee@samsung.com>
Date: Wed May 23 15:00:21 2018

notify a deprecation message for anonymous getter

notify a deprecation message for HTMLFrameSetElement's
anonymous getter to user before remove it.
this feature will be removed at m70.

Bug:  695891 
Change-Id: Iae0c0848bed691d2b3cc472a1c515c3e926c176c
Reviewed-on: https://chromium-review.googlesource.com/1070170
Commit-Queue: Hwanseung Lee <hwanseung@chromium.org>
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561083}
[modify] https://crrev.com/b7a3fd5d28f223b18d5d8660aa57ce190678b921/third_party/blink/public/platform/web_feature.mojom
[modify] https://crrev.com/b7a3fd5d28f223b18d5d8660aa57ce190678b921/third_party/blink/renderer/core/frame/deprecation.cc
[modify] https://crrev.com/b7a3fd5d28f223b18d5d8660aa57ce190678b921/third_party/blink/renderer/core/html/html_frame_set_element.cc
[modify] https://crrev.com/b7a3fd5d28f223b18d5d8660aa57ce190678b921/tools/metrics/histograms/enums.xml

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6a866d29f4314b990981119285da46540a50742c

commit 6a866d29f4314b990981119285da46540a50742c
Author: Hwanseung Lee <hs1217.lee@samsung.com>
Date: Thu Aug 09 01:58:53 2018

Remove HTMLFrameSetElement's anonymous getter

we have been notifying HTMLFrameSetElement's
anonymous getter will be remove since m70.
m69 already branched out.
and now it is time to remove this.

Bug:  695891 
Change-Id: I145573c2a5c939181c679809531786c975820490
Reviewed-on: https://chromium-review.googlesource.com/1064250
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Hwanseung Lee <hs1217.lee@samsung.com>
Cr-Commit-Position: refs/heads/master@{#581739}
[modify] https://crrev.com/6a866d29f4314b990981119285da46540a50742c/third_party/blink/public/platform/web_feature.mojom
[modify] https://crrev.com/6a866d29f4314b990981119285da46540a50742c/third_party/blink/renderer/core/frame/deprecation.cc
[modify] https://crrev.com/6a866d29f4314b990981119285da46540a50742c/third_party/blink/renderer/core/html/html_frame_set_element.cc
[modify] https://crrev.com/6a866d29f4314b990981119285da46540a50742c/third_party/blink/renderer/core/html/html_frame_set_element.h
[modify] https://crrev.com/6a866d29f4314b990981119285da46540a50742c/third_party/blink/renderer/core/html/html_frame_set_element.idl
[modify] https://crrev.com/6a866d29f4314b990981119285da46540a50742c/tools/metrics/histograms/enums.xml