What Garmin devices work with Slopes?

You will need to use a Garmin device that has the dedicated activity profile for skiing/snowboarding. If you're not sure if your watch has this functionality, you can check on their website.

On the website you find your device → Specs → scroll down to "Outdoor Recreation". If you can use it for skiing/snowboarding, these activities will be listed in "Available Outdoor Recreation Profiles". Alternatively you can look up your device's details via their support center.