The direct answer
Compare the list of accounts you follow with the list of accounts following you. The difference is the current not-following-back group.
The check uses your export and never needs an Instagram login.
The three useful views
GroupSet ruleExample
MutualFollowers ∩ Following475
Not following backFollowing − Followers208
You do not follow backFollowers − Following1340
What to do with the list
A one-way relationship can be completely intentional. Creators, news sources, friends, and professional contacts do not all need the same rule.
Use the result as a review queue, not a command to remove people.
About “unfollowers”
A single export is a current snapshot. It can prove who is absent from today’s follower list, but it cannot prove the date or motive of an unfollow.
To identify a removal between dates, save a baseline and compare a later export.