Yes, they definitely can see your future posts unless and until your account is not a private account. Apart from seeing your posts here, I have come up with the list of things which happens when you unfollow someone on Instagram:
You will no longer see their posts
Right after you unfollow someone on Instagram you stop seeing their further photos, stories or video posts. To see their future posts without entering their profile you have to follow them again.
They won't get notified
Just after you unfollow someone on Instagram, the individual you unfollowed will no longer get notified with any type of activities you are doing on Instagram like posting stories or going live, but if you follow them again, they will get a notification that you followed them.
Their number of followers will get reduced
Your unfollow action will lead to in reduction of a number of the follower of that particular user. Well, there is no notification made for such an action.
You will disappear from their follower's list
Just after you unfollow an individual on Instagram, you will also disappear from their follower's list. If they check their follower's list, they won't find you, and as a result, they might realize you unfollowed them.
They can still message you
A person you unfollowed on Instagram can message you. The messaging function doesn't get affected even after you unfollowed someone on Instagram.