It surely wasn’t included in the default installation of CI since CI ain’t no CMS or alike but a framework for helping developing apps. However I’m surprised that you lost this authentication module when upgrading because - I guess - you just overrode the system files and replaced names with the same filename but did not delete any files ?!
create_your_own from scratch.
I prefer the latter.
I will go with ‘create_your_own from scratch’ because I have already created user and permissions management. Tank_auth and ion_auth looks pretty robust but is too late. Good to know for future reference