I had to make an account just to reply to this. I've been using one for the longest time called 'WhatsApp Tray' that can be found on GitHub. Recently however, Whatsapp updated their app and removed all downloads to their non UWP (I'm assuming) app, thus forcing you to download from the Windows Store. Unfortunately it means the old app, which was working till yesterday, just stopped working and was stuck in an infinite loading loop whilst screaming at me to update. As you might have figured, the 'WhatsApp Tray' app which hasn't been updated since Jan 2022, just doesn't work with this new version.
Thankfully! If you're still looking for something to minimize whatsapp to tray (doesn't work on startup as of yet, unlike the old one) there are alternatives available that I've found. They are the following:
I've tried out RBTray and PortableHideWin, and while both are good, they're not quite what WhatsappTray was. I haven't tried MinimizeToTray, Traymond, or KSTray, so if you end up trying each one, tell me which is the better of 'em will ya.