1 You stop seeing their pictures
When you unfollow someone on Instagram, you stop seeing their pictures in your feed. In order to see the pictures, you will have to follow them again.
2 They still see your posts
The people you unfollow on Instagram can still see your posts normally provided that they are still following you.
3 They won't get notified
When you unfollow someone on Instagram, they won't get notified, but if you follow them again, they will get a notification that you followed them. See what happens when you unfollow someone on Facebook.
4 Their follower count will get reduced
When you unfollow someone on Instagram, their follower count will get reduced. If they already knew how many followers they had, they might conclude that someone unfollowed them. See also why do people unfollow you.
5 You will disappear from their followers list
When you unfollow someone, you will also disappear from their followers list. If they check their followers list, they won't find you, and as a result, they might realize you unfollowed them.
6 They can still message you
The person you unfollowed can still message you normally. The messaging function doesn't get affected by the unfollow action.
7 You might have to request authorization again
If you unfollow a person then decide to follow them again, you will have to wait for their authorization if their account is private.
8 Your likes won't disappear
When you unfollow someone, your likes and comments will remain there and they won't disappear.