No, your friends won't receive any notifications when you remove them from your friend list. This action is discreet, and they won't notice any change in their app. They can continue sending you letters, but these letters won't appear in your inbox, and their status will remain "Unread." Additionally, they can still access and read the letters you've previously exchanged and view your profile as usual. In short, removing a friend is a private action, and they won't be aware that you've removed them.
Please note that the "Remove User" function in Slowly is equivalent to the block function. Once you remove a user, they won't be able to send you letters or match with you again in the future.