How to remove KDE Connect app icon from Notification bar?
Excerpt taken from KDE Connect – New stuff 0x3:

Android Oreo introduced some restrictions in regard to apps running in the background. In the future in order to be able to run in the background KDE Connect needs to show a persistent notification.
To hide the notification you need to long-press it and switch it off. Other notifications from KDE Connect are unaffected by this.