If you are already using a database and think you might want a simple index (ex: on an ID) then sure, just add it. But if feeling like you should have an index pushes you to start using a database, or if you want the support something complicated like full text search, then I don’t think it’s so clear.
(This post is not anti-index, it is anti-”you should never be doing full table scans in production”)
I see. That makes sense.