The requirement for User Email Verification was enforced with the Summer '24 release.
There is no native Salesforce process or functionality that would be expected to set a previously verified User's email address to unverified outside of user record email address modification.
Users created prior to the Summer '24 release may not have verified email addresses. Verification of user email addresses is required when:
Check the following resources to identify why a previously verified user email address may have become unverified.
See User Email Verification for available methods to verify user emails.
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