I recently started using i3 (my first WM), and I used to use Plasma. In Plasma, when Spotify was opened, I just went to my system tray and there was a Spotify icon, which if I right clicked it, it let me close Spotify to system tray.
In i3 the only tray icons are the programs that are actually in the tray (Steam, e.g), so I cannot “close it to tray” as in Plasma.
I do exactly the same, I usually just move Spotify to my 9th workspace and I let it there, but recently I have been noticing a spike in GPU usage specifically done by Spotify, and I want to close it to tray so it doesn’t use these resources.