Scanning with phpstan identifies some straightforward fixes:
Line web/modules/contrib/alternative_user_emails/alternative_user_emails.module
68 Call to function in_array() requires parameter #3 to be set.
68 Construct empty() is not allowed. Use more strict comparison.
73 Call to function in_array() requires parameter #3 to be set.
73 Construct empty() is not allowed. Use more strict comparison.
74 Call to function array_keys() requires parameter #3 to be set.
91 Function alternative_user_emails_entity_query_user_alter() has no return type specified.
110 Function _alternative_user_emails_expand_mail_condition() has no return type specified.
121 Call to function in_array() requires parameter #3 to be set.
Fixed
1.0
Code
The MR contains (to the best of my knowledge) very safe additional type restrictions.
Automatically closed - issue fixed for 2 weeks with no activity.