I think we can - CL https://codereview.chromium.org/117263002 which landed 3 years, 5 months ago, had code to migrate from that pref to a different pref in local state, so any profile logged in in the last 3 years should not have that pref anymore.

CL: https://chromium-review.googlesource.com/c/530848/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/2e9f1b26e22fa66124b13ed4317a361c3694c691

commit 2e9f1b26e22fa66124b13ed4317a361c3694c691
Author: Pavol Marko <pmarko@chromium.org>
Date: Mon Jun 26 10:10:42 2017

Remove kUsedPolicyCertificatesOnce profile pref

This CL removes the kUsedPolicyCertificatesOnce profile pref. It has
been replaced by the kUsedPolicyCertificates local state pref in
https://codereview.chromium.org/117263002.

BUG= 731100 

Change-Id: I7874e143410957905c384732bcd0a1c24a15bffc
Reviewed-on: https://chromium-review.googlesource.com/530848
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482224}
[modify] https://crrev.com/2e9f1b26e22fa66124b13ed4317a361c3694c691/chrome/browser/chromeos/policy/policy_cert_service_factory.cc
[modify] https://crrev.com/2e9f1b26e22fa66124b13ed4317a361c3694c691/chrome/browser/chromeos/policy/policy_cert_service_factory.h
[modify] https://crrev.com/2e9f1b26e22fa66124b13ed4317a361c3694c691/chrome/common/pref_names.cc
[modify] https://crrev.com/2e9f1b26e22fa66124b13ed4317a361c3694c691/chrome/common/pref_names.h

Done. Please note again that this was about removing a migration path from an old pref to a new pref.

The only reasonable verification I can think of would be verifying that the policy-provided trust root used -> multi-profile disabled logic still works fine as a regression test.