I spent a few minutes on the site trying to see what was supported and saw nothing. I have a passing awareness of KDE so I could assume that the desktop platform was Linux, but the mobile platform was not specified anywhere in my initial searching. I have to admit I didn't dedicate a lot of time to that though, since I have no interest in running desktop Linux.
KDE connect works on macOS too.
Which actually makes the information lack even worse since there was no indication of what desktops and phones were supported in my cursory examination. It always amazes me how often basic information like this is absent from the promotional material for various products.
When you click Download you are given the list of supported platforms.