Script code to find out who among your users in your Google WorkSpace systems is using an alias:

Just go to script.google.com and run the code below. That’s it. 🙂

function listUsersWithEmailAliases() {
let pageToken
let page
do {
page = AdminDirectory.Users.list({
customer: 'my_customer',
maxResults: 100,
pageToken,
fields: 'users(name/fullName,primaryEmail,aliases),nextPageToken',
})
let users = page.users
if (users) {
for (let i = 0; i < users.length; i++) {
const user = users[i]
if (user.aliases && user.aliases.length > 0) {
Logger.log(`User ${user.name.fullName} <${user.primaryEmail}> `
+ `has ${user.aliases.length} email alias${user.aliases.length > 1 ? 'es' : ''}: `
+ JSON.stringify(user.aliases))
}
}
} else {
Logger.log('No users found.')
}
pageToken = page.nextPageToken
} while (pageToken)
}

You can then use the following GAM to bulk delete these aliases

gam redirect csv ./OldDomainAliases.csv print aliases aliasmatchpattern ".*@olddomain.com" onerowpertarget suppressnoaliasrows
gam redirect stdout ./DeleteAliases.txt multiprocess redirect stderr stdout csv ./OldDomainAliases.csv gam remove aliases "~Target" "~TargetType" "~Aliases"

https://github.com/taers232c/GAMADV-XTD3/wiki/Aliases#remove-aliases-from-a-specified-target