Hello ShashankSaxena-2458,
Thanks for your question.In my experience, the typical behavior is it will truncate it. However as it is an attribute that will be used it will end up failing on target system. It is always recommended you be within the limits. Please note for the username or UPN attribute consideration includes the domain.
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola