OOF will only work once. Until OOF is disabled and enabled again, users will not receive OOF again.
So, I would suggest you use transport rule to inform users directly(Please note, emails will cannot be sent to old domain):
If you want to use OOF, you can use command below to enable OOF for all mailboxes:
Get-Mailbox | Set-MailboxAutoReplyConfiguration -AutoReplyState Enabled -InternalMessage "Dear Sender, the old domain will be abandoned, please contact us with the new domain(myName@newDomain.com)." -ExternalMessage "Dear Sender, the old domain will be abandoned, please contact us with the new domain(myName@newDomain.com)."
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