ENG-10749.
First attempted in D14417 but we didn't decide to do it then.
Now it's safe:

• versions without Identity are now unsupported by keyserver, so the user would be logged out if on a very old app version.
• scenario of coming-back users is handled by LogOutIfMissingCSATHandler and the "claim username" protocol during login.
• it's very unlikely that a coming-back user from pre-Identity times opened the [updated] app and was still logged in.

Manually wiped the store.commServicesAccessToken to confirm I'll be logged out.